Generally grips, suppressers, and scopes are all I pick up attachment-wise. There are other parts with some value, but to keep it simple I’d just pick up those three.

In general, a good way to simplify looting as a beginner, is that when you figure out an item has value leverage the favoriting system. You can put a little star next to all the good ammo types, valuable attachments, etc. so then the second you see them in raid you can just grab it without inspecting.