Hooooookay.
1) any thing that is AR based, but not 416/417/SR25 for example. Some are specific to things like MWS or PTW so watch out for that but as long as it says AEG compatible and isn't off an AK you should be fine.
2) the original nut is most likely specific to THAT handguard but any decent handguard should come with its own barrel nut (unless you're going old school and want a delta ring arrangement but I doubt that). Also, if you buy a G&P barrel make sure it isn't a taper fit one or you'll need an extra adapter.
3) yeah, let us know how THAT goes! /jk
4) what OEM stuff are you meaning?
If you need a Dremel to fit a handguard to (basically) an M4, you done fucked up somewhere. You MIGHT need a specific tool for the barrel nut if you get something like a URX4 but that's a whole different story.