Man, I can't believe no one has replied. I have not built one from scratch but I have several. I personally like the Bushmaster line, they are a little cheaper than the others with great quality. Bushy's, DPMS, Rock River, Armalite all interchange. I might have my wires crossed, I would have to look but I think Colt uses a small takedown pin where the ones mentioned above use the larger diameter pin. I have complete rifles in different configs by the manufacturers listed above. I can swap all around but I never change the config of the Colt's. Rock River may be a little cheaper than Bushmaster but I am not sure of the quality yet. I have a heavy barreled upper that I just bought and have shot on a Bushy lower and it groups real good.
AR15.com has alot of info and those guys are ARnuts.
Hope this helps ya.