You'd best wait for more experienced hands than mine, but I can't resist a comment. I've heard from several sources including our own Jeff Carpenter:
Buy a quality rifle / carbine and don't try to build one. In the classes that I have been to approximately 90%+ of the "built" guns have gone tit's up.
I have an RRA gun, got it early this year (that's how new I am) and have had no major problems with it.
A shooting buddy had one of the hotshot dealers build him an AR especially for 3 gun matches and he is constantly cursing it for failure to feed and eject.

So why are you building a gun from so many components?

Maybe some of the experts here can shed some light on the next question for me:

Which is more likely to cause him trouble - the mixed-brand bolt group & upper or the mixed-brand lower & upper?