I'd go Colt at just under 1k but if you want even cheaper, then the smith & wesson or aero would be the only viable options imho. Way too many qc issues with psa to considerFrom my experience:
Palmetto State Armory (PSA) - $450 - great starter gun to learn on
Aero Precision M4E1 (AP)- $950 best bang for your buck under $1k, great quality for the price
Larue Complete Upper - $1,200 - I only have 100 rounds through it and I thought it was okay
Bravo Company Manufacturing (BCM) - $1,400 - Quality rifle to trust your life on
Geisssele Super Duty - $1,600 - Quality rifle to trust your life on but slightly better than BCM. Just slightly.
Radian Model 1 - Quality Gucci build at its best - $3,000
To be honest, I'd say Aero Precision, Bravo Company, and Geissele get my vote for best quality at an affordable price. I think the Aero and the Bravo Company are the best value obviously, and you can trust your life on all three of these guns.
Just buy a lower first, even a complete lower if you don't want to build one from scratch, from between $150 (from PSA), $350 Aero, to $400 from BCM, and then save up money for your complete upper, and a BCG (if it doesn't come with one).
I'd go Centurion Arms for 1k-2k
anything over, KAC/LMT
for the life of me I can't understand the love for Geissele; way too overpriced for what you get and known qc issues