in listening to eric cortina(personally shot against him in years past) First place to spend $ is the barrel. if the barrel is not accurate, nothing else will make the gun accurate.
After that, some things matter more than others. IMHO, All the upper does is present the bolt to the barrel. Lap the barrel to the upper and that's about it. You can spend crazy $ on triggers, but can YOU tell the difference? there are lots of great and reasonable triggers and money not spent here can go to better quality glass. So much is like fishing lures. the intend is more to catch fishermen than fish