For what it's worth (yes, yes....I'm labeled as the Armalite pusher).....Armalite is running a damn good sale (you can't buy the parts individually for what they're asking) on their complete uppers right now. You can get into one for just under 600. Then, get yourself a lower (between 300 and 400 already built)......and you can customize from there. Leaves you plenty of room.
Just throwin it out there.