If you have Wings over Europe, which I'm not certain of, I would recommend the "all in one" NATO Fighters 1 and 2 add ons. Lots of stuff with minimal hassle.
Unfortunately, there hasn't been a lot of bundling like that because people have gotten all pissy about their stuff being with someone else's or whatever, so often you have to do everything separately yourself.
First Eagles is the same type of situation, with little bundling and most stuff separate.
I'd say the easiest things to do are usually just adding planes. The 2nd easiest is terrains, followed closely by the Weapons Pack for the SFP1/WoX series which is required if you want to use the weapons on a lot of the add on planes. Without it, you may have a plane with nothing but a gun.
Of course, for Il-2 the only addons are commercial or just skins, nothing complicated there--unless you buy multiple addons and then there can be a dance as to which ones get installed when and how in the patching process...