You need to determine what you're going to use the AR for and your budget. Hard for us to give recommendations until then.
Nothing wrong with buying complete uppers and building your lower.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek