I took some time to put this together mostly for my own info, but thought I would share it out. This is from my own observations and testing and the info may not be 100% correct. Feel free to correct any mistakes. I think the most likely mistakes in the tables are which affixes are prefixes and suffixes for sure. Headshot Damage and Crit Damage seem to be both on some item types for example.

First off i am going to post a couple of tables of affixes found as secondary stats on items. All gear that isn't the Claw Hammer, Wrench, Drill, or Fishing Poles and can roll higher than common "grey" variants can roll secondary stats. The rarity "color" determines how many affixes can be rolled on a given item. The item type determines the types of secondary stats that can be attached. This is a very similar setup to most RNG loot based games such as Path of Exile or Diablo games. First here is the table of amount of affixes by rarity.

Affixes are broken into Prefixes and Suffixes in this game. The most an item can have is 3 prefixes and 3 suffixes. An affix can roll multiple times. As far as min roll vs max roll of the numerical value, the affixes do not have ranges. It is an either or scenario. You either get the bad minimal roll or the good maximum roll when there is a difference. Not all affixes I have seen have different rolls so the min and max will be the same. When you see a value on your gear with a different value than what is listed then it had a multiple roll and it combined the values of both rolls.



The only thing about the table above is I am not sure there is a "min" amount of affixes. This is a guess only because I have never seen a unique with less than 2 affixes. If there is no min, then the other working theory I have is that there is a "dud" prefix and "dud" suffix which are literal blanks that have no stat.

In the tables below are my own opinion on rank weighting for stat usefulness per category of item. Here is the break down.

S = Must get.
A = Great to get.
B = Good overall and will work with any build.
C = Niche stat. Somewhat good, but may not be useful for every build.
D = Mostly bad. Doesn't tend to affect builds or help character performance much.
F = Very bad. Waste of a roll.

General Affixes that can apply to all items.

Weapons

Shields

Armor


* Coils sockets also seem to have an extra roll. Look at the screen shots below. The default socket count for a head armor piece is 1 socket. That last screen shot is of a helmet with 7 sockets and 2 prefixes. Which means it had both suffixes roll a 3. Seems to be extremely rare, but here is link to image.

4 affixes and No coil rolls = https://i.imgur.com/3fZfTGg.png (1 default coil socket)
3 affixes and 1x coil single roll = https://i.imgur.com/1Wgcnkr.png (2 coil sockets)
2 prefixes and 1x coil double roll = https://i.imgur.com/7VPOFNv.png (3 coil sockets)
2 prefixes and 2x coil double roll = https://i.imgur.com/Iy51n3g.png (5 coil sockets)
2 prefixes and 2x coil + 3x coil roll = https://i.imgur.com/1nw4ABX.png (6 coil sockets)
2 prefixes and 3x coil double roll = https://i.imgur.com/bBm6gyD.png (7 coil sockets)

Accessories


* General Crit Rate suffix isn't applied to accessories as it has it own crit rate prefix as an override.

I will also like to say that affixes have a "weight" to them in how common vs uncommon they are. For example, Crit Rate is pretty rare for an affix on anything, and I've only ever seen a double roll of it to a 10% once because of how rare it is. I've seen both double rolls and triple rolls on things like Poison Reinforcement which is a more common affix. I have no way of deducing the weighting system, but letting everyone know I believe it exists based upon my testing thus far. I am pretty sure there is a different weight for min vs max roll as well unless those are registered as separate affixes. Which might explain why I have trouble determine a couple of these being a prefix or suffix. If the min roll is a prefix and the max roll is a suffix that adds a little complication. Unfortunately at this time I can't verify that. Crit Rate is also funny because it is the same on everything with a 5% roll except on accessories which can roll a 2% version. So I believe that is a separate affix specifically for accessories.

As mentioned in the OP, Headshot and Crit damage are strange ones. First off, they can appear on any item, but are different values for armor vs anything else. For armor it is always a 15% roll where it can be a 10 or 20 roll on anything else. They don't seem to follow the same pattern of being always a suffix or prefix either with how they show up in the order and with other affixes on different items. There may be a few more like that I missed.

Finally if I had to give me opinion on what to look for in affixes to try to get here is what I would say.

For weapons, Flat damage > % damage since flat damage is usually higher value than the percentage.

For armor evade is one of the best affixes you can get.

For accessories the stats to look for are evade, invincibility, and damage leech are the best defensive affixes you can get. Two of which are only available on all accessories.

All stats > single stats. For single stats Memory > Strength > Vitality > Endurance > Judgement > Attention.
Crit Rate is always good to get on anything you can find it on.

Wound Reinforcement is king for damage on any item you can get it on. Get as much of it as you can along with Crit Rate to maximize DPS. After that Crit Damage, and Headshot Damage are good for additional damage bonuses. Fire Reinforcement is fine for flamethrowers, explosive for grenade launchers, and lightning for plasma weapons. If a specific weapon is of a specific damage type then the reinforcement affix matching that damage type is a decent bonus. However, the damage from coils is worthless to bump up with affixes. For example, fire dot damage from the ignite coils is boosted by fire reinforcement, but the damage of the dot is so low that it isn't worth building around it.

Anything else you get for an affix for the most part is just meh usually.