Bravo Company makes some top notch parts. Snag a lower and an Upper from them, slap on an optic of your choice in a LaRue Tactical mount, and you're good to go.
This is about all you need to know, poke around, deals are offered you can get Bravo company Midlength gas system 16" rifle uppers for about 700 bucks, and you should never have any problen if you keep it lubed.