I've build 2 AR's now. Started with CMMG lowers and Rock River Arms everything else, with the exception of a Colt Carry Handle and Knights Armament RAS system, covers and broom stick.
Go with the Major Quality Manufactures... I suggest you look to who has Gov't Contracts, ie; Bushy, Rock River, CMMG. I've seen alot of AR's, the most recommended upper to me was Rock River & Fulton Armory.
If you can, use the same manufacture for everything, it will make the rifle worth more in the long run, instead of considered a "build" gun.