X2!
I've put together a bunch of uppers and complete rifles over the years. I never had a problem with any of them because I used quality parts. I happen to have a full set of headspace gauges for bolt gun builds, all uppers checked out perfectly.
As for the OP, not to rain on your parade, but given the nature of the questions you asked I think you'd be better served by purchasing a complete rifle and throughly learning the platform and how it works before taking on such a project. In any event, best of luck.