For your questions:

- HOF mode gives all enemies a significant buff, the most noticeable being their increased hitpoints. In HOF, all enemies get their HP multiplied by 3 and then a flat 80 is added on top of this. This makes HOF kind of a slog since all enemies turn into damage sponges... and it's mostly noticeable in the early game where tripling a goblin's HP is not THAT big of a deal (it goes from 6 to 18) but then the flat +80 takes them to 98, so you an probably see how HOF is a lot slower than the other difficulties.
On the other hand, all experience points you gain are doubled (as per insane difficulty) and then a flat 2000 is added. So you will also gain levels at a much higher rate than normal.

- IWD is pretty difficult when you play it for the first time. The encounter designs differ heavily from Baldur's Gate: there are a LOT more enemies and in most cases they are not prone to split up if you try to edge up to them at the edge of your range of vision. If one enemy spots you, he will usually bring his friends and extended family with him.
But just like in most RPGs, with some metagaming knowledge, the game can turn very easy. I would recommend playing without mods on your first playthrough and see how you like it.

- Party composition will depend a lot on your personal preferences and also on what party size you are going for. HOF can be soloed starting with a level one character, but it's very slow and something you'd do for the sense of accomplishment rather than for an enjoyable game. But the larger your party, the more characters split the experience gain, making for slower leveling. If you want a fairly power-gamey party, I personally enjoy the following:

(All of good alignment for the cleric buff Righteous Wrath of the Faithful)

1. Gnome multi Fighter/Illusionist - to be your tank when he's got his spell protections up and running.
2. Half Orc multi Fighter/Cleric - buffer/healer/insane melee damage
3. Human dual Fighter7->Cleric - basically a copy of the above which sacrifices early game cleric abilities but ends up even more powerful
4. Human dual Fighter7->Druid - same concept as above but with druid spells, of which there are some really good ones in this game
5. Elf multi Fighter/Thief - Starts as a ranged fighter and switches to melee when the available melee weapon choices overtake the ranged ones. Handles all traps and locks.
6. Half-Elf Bard (no kit) - ironically your main arcane spellcaster, and the vanilla bard's songs are excellent in IWD.

You mention that you like to have a pure spellcaster, so you could also consider changing one of the F/Cs, the F->D or the bard for a sorcerer. The sorcerer is a better choice than a mage in IWD due to how scarce spell scrolls are.

Originally posted by Amos:

For your questions...

Hmm... It's seems that IWD has a larger party size than BG? Well ofc my playtrough will be among the full party. There is appear to be existence of recruitable character through the story progress like in BG, or just making party on the start and viola?
Interesting choose of the classes, I will take it as a basis and encourage change it to more single-role like, hope it will not ruin HoF. Actually I have played BG with mods that making enemies attacking in groups, I had two mods with fully hardcore setting: Swordcoast Stratagems and another one, forget the name. It's actually was hard... :/ Main problem was to achieve megatons of supplies such as potions, scrolls, explosives and so on, thousands of reloads after party falls, playtrough was not very enjoyable because of that but very fun in the end, I hope HoF will be something like that!

For now I am trying to create custom portret for IWD, as I have for BG. The chars cannot be transported so I better make new, in sake of gender equallity it will female magician (the last one was male), but don't know what kind of mage, I played Half-Elf Diviner (a super mage was), dunno what with magic in Icewind... Too poor that orcs cannot use magic properly...

Originally posted by Amos:

In IWD, you create your full party of six at the start of the game. Unless you are using mods, there are no recruitable characters along the way.

OMG! It's so stupid, but anyway... So the six chars from the beginning will be. About magic, OMG too! This IWD is quite different of BG!
Well, the hell with two opposing schools, no matter, I mean about the main char, I building my choice not much of the effectiveness but on a fairy-tale, a story... But of course I can create biography about a generalist, for an example she will be a graduate of general magic studying in the university of somekind and then for some reason travel to the bloody Icewind Dale. The specialist mage opening some stuff for story — for an example, she was a destructive type of magic user and after the accident was forced to move into Dale, where get involve along with companions that she recruit on travel into an adventure! The Wild Magic is actually fun idea but I think it's too insane for character with Lawful Good, maybe for Chaotic Good, or Chaotic Neutral... Indeed the choice is hard!

The main problem now is to define what kind of mage she will be, race, then create custom portrait for MC, a backstory, and properly place that junk into the game!

P. S. I am very slowly learning digital art and storycraft, with an insane idea to achieve some money somehow on this field.

IWD grants increased XP for the higher difficulty modes. If you are curious, Heart of Fury mode gives most XP, and with a capable party, you can start at level 1, too.

There are unique events for a paladin (or kit) and a bard (or kit). The integrated Heart of Winter expansion offers a good theme for a barbarian, but specifically creating one for that expansion is overrated.

Reputation is not a big thing in IWD. Alignment leads to a few unique choices with/without affecting reputation. Alignment also governs whether you can use some items.

The base game features a point of no return. You are well-advised to start the expansion(s) much, much earlier - perhaps travel forth and back a few times. There are some restrictions, so you cannot return to the base game world 100% freely at any time.

With regard to MC and backstory, it would need to cover for 1-6 characters depending on how many characters you create. In IWD's story, you play a typical party of adventurers that get hired at least once and then get pulled in into the story development.

Originally posted by D'amarr from Darshiva:

The base game features a point of no return. You are well-advised to start the expansion(s) much, much earlier - perhaps travel forth and back a few times. There are some restrictions, so you cannot return to the base game world 100% freely at any time.

Just to clearway, I can start expansion Heart of Winter, in the middle of the game, and then take back to the original campaign?

Originally posted by D'amarr from Darshiva:

With regard to MC and backstory, it would need to cover for 1-6 characters depending on how many characters you create. In IWD's story, you play a typical party of adventurers that get hired at least once and then get pulled in into the story development.

I mean I will create by myself only a single character, which will be the main character (MC). It's magician female will be, but dunno what specialization and race, and then I will make a "BIOGRAPHY" section for by myself too. All other character will be already pre-made that already exist in the game, such as Kirika or Cadriel, they obviously has their own biographies.
OMG! In IWD so annoying mage specialization restriction based on race... >:( If I am mistaking not, in BG were much more freely situation? LOL. But Necromancer can be Lawful Good with human race... :D Well, I suppose I can imagine such situation.

Originally posted by frostyshad:

Just to clearway, I can start expansion Heart of Winter, in the middle of the game, and then take back to the original campaign?

There are two ways how to play the expansion:

1) It is available on the game's title screen where you can choose between the base game and the expansion and then either create a new party or import an older one.

2) The expansion content is integrated into the base game. In the village of Kuldahar you meet an NPC that will tell you about the expansion. You must be level 9 at least before you can agree on being taken to the expansion's starting village. Within the expansion areas, the same story NPC can take you back to Kuldahar, so you can continue with the base game before deciding on wether to return to the expansion stuff again. The only catch is that the NPC is not available all the time for story reasons.

The second expansion Trials of the Luremaster is integrated into the Heart of Winter expansion.

Originally posted by D'amarr from Darshiva:

The expansion content is integrated into the base game. In the village of Kuldahar you meet an NPC that will tell you about the expansion.

It's awesome! Good idea about such type of gameplay with expansions was. The custom portrait is ready! It's has been made from three other images from portrait pack. But now I have some more question...

— Which character is better to replace in the party based on effectiveness in the game with my own MC? The variants is next: Corris (Berserker), Baern (Cleric/Ranger), Cadriel (Undead Hunter), Kirika (Fighter/Thief), Ilauna (Mage Generalist), Felicia (Bard)?
Personaly, depends on their biography, I will disband Ilauna, or Corris, or Cadriel. The first one is looking for a fun adventure, no more, the second two have the same biography... Somebody of game developers bothered to make a different one for this two... :S

— What magic specialization better to take for my MC?
She will be Elf, so the only two choices is Diviner or Wild Mage. The first one is proved to be very strong on hardcore settings in BG/SoD/BG2, the second one is quite opposite... I remember some Wild Mage in BG, "Naira" was the name or something, and this mage was a total disaster on hardcore settings, so I disbanded that char from party... There is little things in BG that disappointing more, than in the end of difficult battle with tons of reloads, your party at last suppressing some, I don't know, Gith-Yankie in the closed room, and then you suddenly got a fireball explosion in the center of a group and some chars without protection falling like a grapes... But the point I playing a lot for Diviner, maybe I should try another magic school? I mean, maybe Wild Mage is not so bad?

Well, it's nevermind, I have already started HoF settings with the Wild Mage... And no way back left.

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2380808961
I feel I will regret it, but the Wild Magic so good correlated with MC backstory about the magician whoes fate are doomed and who seeks some place to leave in peace. Also I will try to make my own GUI mod for IWD: EE to make black font on whitish background.

Originally posted by frostyshad:

— Which character is better to replace in the party based on effectiveness in the game with my own MC? The variants is next: Corris (Berserker), Baern (Cleric/Ranger), Cadriel (Undead Hunter), Kirika (Fighter/Thief), Ilauna (Mage Generalist), Felicia (Bard)?

Uh oh, starting a HoF mode playthrough at level 1, you will need massive fire power of any sort, most efficiently are ordinary ranged attackers and later AoE/missile spells. You cannot rely on tanks doing their job, not with weapon'n'shield and not with support either. The bonuses from bard song are neglectable due to HoF mode buffs, and a sorcerer learning a good choice of spells can give better support.

Originally posted by D'amarr from Darshiva:

Uh oh, starting a HoF mode playthrough at level 1, you will need massive fire power of any sort, most efficiently are ordinary ranged attackers and later AoE/missile spells.

Or it's can be replaced with enormous amount of reloads. I have won the battle against goblins near starting village with my party only because Wild Surge summoned five goblin Elites which blocked the bridge with enemy goblins attacking. The point the Wild junk have changes the gender of my magician, I hope it will not break the game? Because I don't want to repeat those battle... And actually tanks with -1 AC (Splint Mail) + Heroic song + Bless fights fine against single goblin, just need enormous amount of healing, which is problematic, Druid has only ~4 healing spells. Goblins using probably leather armour because arrows and daggers works much more better blunt weapon.
Good thing in IWD compared to BG that military supplies cost negletable cash. And resting in the tavern too, only 3 gold for 10+ HP restored.