As stated earlier a real M14 receiver is worth big, big bucks. There is an action bar and selector switch that you will not see on the commerical receiver. These parts allow a real M14 to operate in a full automatic mode.
The Kelley's attempted to make a look a like version of the M14 with their receivers. Their receiver had a dummy selector switch and a modified action bar. The receiver could not be easily modified to full automatic, but these features were too close for the comfort zone of the BATF. They confisicated all of these receivers. I doubt anyone is going to try and make another look a like for a while.
Outside of the full auto parts the actions the civilians use have all the same basic parts, only differing in small details.
A M1a (generic term) is an entirely functional, accurate and reliable rifle.
If you plan to build one, well I recommend using as many GI parts as you can. New condition GI parts have served me well. I started with Springfield Armory receivers and used all GI parts (except for match sights, stocks, barrels and recoil spring guides) and I have never had a problem with my rifles.
The topic of who makes the best rifles or receivers is something that will cause much discussion.