While not strictly needed, there's also no downside. Texture Upgrade Pack - This improves all the in-game textures. It bothered me the first time I played, and while it's a small thing, I think it makes the game better.

and also something you could take from their inventory. SCP's Shotgun Mod - This fixes a problem with one of the enemies where their weapon was part of their corpse. These aren't changes for the sake of making changes, they're legitimate improvements on the experience. I respect the hell out of the SCP's creators, every change they made makes sense in the context of the game and it's lore. In vanilla SS2, it's possible to spend your skill points 'wrong', SCP reduces this somewhat. Did I enjoy playing without it the first time? Sure, but why invite a bunch of jankiness to your first playthrough when it can all be patched out? It also reballances a few things regarding weapon damage and skill trees to make most builds more viable. Vanilla shock (even patched) has a bunch of glitchy nonsense in it that SCP corrects. SCP - The shock community patch makes a TON of changes to the game, but they're all done with the original spirit/intent of the game in mind. Personally, I recommend a set of core mods that I think make the game legitimately better without changing the 'spirit' of the original:

System shock 2 subtitles