Steam help: Checking updates/DLC and mods

How do I check what version of a game I have and what DLC I have installed?
I bought the X:Com Slingshot pack since it was on sale but I am close to finishing the original game and don’t want to install the dlc until I do.

There’s some X:Com mods out there (for both the original games and the new one), do these mods work with Steam or how does that work?