Why not go with a built rifle using the same maker and have a complete rifle?
Upper flat top, with BUIS
Complete lower with standard trigger
I believe Legal Trabsfer also sells "Complete" rifles minus the Lower. So you get the Kit and then put the LPK in the lower and you have a complete rifle.
Rock River Arms does the same thing. I've been "eye ballin" the above STAG setup, but I want the complete upper and build the lower myself with a NM trigger and a Mil spec buffer and aftermarket style stock (ie the new cheap er MagPul stocks that are coming out).