Look around in some of the media threads and see other peoples M-16's. Usually they post what the guts of the gun are and go from there. It's all up to personal preference. Even better keep the gun stock for now, play a few games so that way you can get an idea of how it preforms so then you can figure out what to upgrade from there.
If I was banging all of them, I'd be a manwhore. At the moment, I'm just an opportunist.