... and also understand that yes, most lower receivers are going to work for you and some are significantly less than others. However, as we all know, trigger pull can impact accuracy as well.
If you build your own upper, you'll need a decent set of tools. This doesn't have to be overly expensive but it is an added cost. Proper torquing of the barrel nut is going to also impact accuracy.
As mentioned above, it's hard to beat PSA complete uppers. Throw in a BCM BCG (if you can get one, they sold out yesterday in 10 minutes) or any other FA BCG that's been HP/MP tested (I'm a Spike's Tactical fan) and you should be in good shape.
Grab a PSA blem lower and LPK and you're GTG.