Most of us "builders" buy a complete upper or one minus the BCG and charging handle, then find those parts. Also a quality milspec stripped lower, LPK and milspec receiver extension kit and milspec diameter stock of your choice. Then of course add a sling and rear sight. And front sight if your upper doesn't have the standard A2 FSB.
Lately PSA has been the bang for the buck brand around here. BCM is a great choice and is considered a cut above most others in its price range.