Secrets to AR accuracy? No different than anything else. Barrel, trigger, & you.
In terms of buying, every barrel is like dating somebody new - need to find out what your newest barrel likes. Or, for the married people, what mood just walked in the door.
Personally, I would, without a doubt, build what you want. YOU get to pick what is important to you.
I just finished building a lightweight AR. Why? Because all the rest were heavy barrels. The 24" never leaves the safe. 20" seldom comes with me. 16" always comes with.
1:9 twist will handle everything from 40 gr to 75 Amax. 1:8 twist will stretch all the way to 80 gr AMax and usually 80 gr SMK's.
Personally, go with a 1:9 twist 16, 18, or 20 " stainless barrel. I prefer stainless barrels, but will not turn down a chrome moly, for a good price.
I prefer Billet, machined uppers and lowers, but they are not cheap....and sometimes hard to come by.
Don't get caught up in the "name brand" uppers & lowers. They contract out quite a bit of work to smaller shops, but put their name on them. No big deal, as the quality is still there. Everything is CNC machined now.
Trigger - up to you. I don't mind a crisp single stage. Yes, bone stock, plain jane, single stage trigger.
I like ACE skeleton stocks and plain A2 stocks.
BCG - prefer a nickel boron coated, but would not turn down a chrome. Phosphate coated are last, but work well....but keep the chrome and phosphate coated lubed with a full synthetic grease such as STOS (sold by Ponsness Warren).