Hi, wondering which version you need?
Quick recap...

• W40K: "Chaosbane" released end of May 2019 (*Base, *Deluxe, *Magnus -Edition)
  *Pre-Orders got the "XP Pack" + "Helmets Pack" for free, plus 4 day headstart
  
• Season Pass (incl. with Magnus Edition) was released in Sep 2019
  
• Tomb Kings DLC (Act 5 - incl. with Season Pass) was released in Dec 2019
  
• Mar 2020: 5th class (Engineer) was released as free DLC
  
  
• Nov 2020: new Witch Hunter class was released (NOT included with Magnus Edition!)
  Also, Slayer Edition was added, which includes everything ;-)

Note: on all PC-versions 4K is available for free !!

• (FREE) Skulls Pack - 4 new emotes & 4 unique interfaces in Skull Throne colours
  
• (FREE) 4K Textures - might slow down your game!

• *XP Boost - 5% xp boost
  
• *Helmet Pack - 4 emblematic helmets (Cosmetic):
  
  • The helmet of the Archmage of Yvresse
    
  • A Death's Heads helmet
    
  • The helmet of Ungrim Ironfist
    
  • A Wild Riders helmet

• Gold Boost - 5% gold boost that takes up one of your fan skill slots.
  
• Emote Pack - Taunt your enemies.. includes 4 emotes, one per character.
  
• Pet Pack - 4 pets: Frostheart Phoenix + Ancestor Dwarf ghost + Hunting Hawk + Sprite
  -> To equip your pet, go to skills and then Fan Skills. Pet increases your loot pickup radius.

• Base Fragment Boost - increases your chances of finding basic fragments
  
• Emotes 2 & Blessing - bonus emote for each playable character (6)
  • Blessing: increase your chances of finding rare items
  
• Pet Pack 2 - 4 pets: Flamespyre Phoenix + Ancestor Dwarf ghost + Hunting Hawk + Sprite
  
• Gods Pack - includes:
  • God Skill Tree, which includes new active & passive skills for each playable character
  • Divine Fragments bonus to increase your chances of finding Dinvine Fragments
  
• Tomb Kings - New act with story arc, unique environments and new enemies (Skeletons).
  Explore the desolate land of Nehekhara and fight the forces of the Tomb Kings.
  -> 2-3 hours Story + Bossfight

• Witch Hunter - 6th & newest class. Pistol + Sword

Yes.
- while "Witch Hunter" (Slayer Edition) is only nice to have, it´s also quite weak - aka not needed
- but Season Pass improves Gameplay (Blessing, Bonus FAN-Wheel Skills, Passives) !!
- also removes Grind during Story (leveling all chars to ->50 becomes easy)
- Comfort features (Lootpet increases Pickup-Range)
- Extra Story-Maps are well done and give a nice change of pace