To me, I start with the important things. Barrel must a 1:7 twist, M4 feed ramps, and a chrome lined barrel. A 1:7 twist has been the "MILSPEC" standard, and that's all I own. You can fire every form of 5.56 out there, and it will stabilize the rounds through a 16" long barrel.
Making sure the bolt carrier group is staked properly, and heat treated is another. There are a few companies with enhanced bolt carrier groups (LMT for instance makes one).
In regards to the lower, you don't have to spend hundreds on it, but again stay away from anything polymer. A standard lower parts kit is just fine if you could care less about the trigger. My LMT MRP came with their 2-stage trigger and it is smooth as silk. Not to mention a lighter pull to help with accuracy.
If you're going the build route, there's plenty out there like Delton, Spikes tactical, Daniel Defense, BCM, LMT, Larue etc......
A good AR isn't exactly a cheap process, but you get what you put into it. Learn the basics of shooting one properly, so get a good set of iron sights. The red dots, scopes and all that shit can be added later.