From a "scratch build", I found the Bushmaster Lower parts kit, bravocompanyusa.com, was fine, the DPMS lower kit, cheaperthandirt.com, midwayusa.com, brownells.com perfectly adequate. Shop around, the prices for the same kits can cover a bit of $$$.
One can always "tweak" an AR platform with triggers, firing pins, bolts etc,...
As kpr posted, having extra parts, is a good thing, if you have your heart set on a VLTOR charging handle, molded grip, oversize trigger guard, better bolt, etc,...Save them for trade, repair, or another build.
They are the 57 Chevy of rifles. Just like the hot rods still on the road today. No two are "exactly" the same, but it's still a 57 Chevy, and the internal parts, are 99% interchangeable with each other.
I was set on the Timney, or Geiselle trigger for my first build, but now that I have shot it with the Bushmaster single-stage "stock" trigger, for me, and why I built my carbine, it's more than adequate. Between the Miculek comp. and Enidine Hyd. Buffer, it literally shoots like a .22LR.