As has been said, the barrel is the heart of the rifle. That said, Ballistic Advantage barrels, as used by Aero Precision, are a very good value and very reasonable choice for most recreational shooters.

Anything above about $150 on a BCG (Colt, Daniel Defense, BCM) doesn't really you buy anything other than perhaps some bling or brand bragging rights. I'll spend a few extra bucks on nitride or NiB just to make cleaning go a bit easier and quicker. If I had to buy a BCG today, I'd probably order this Fail Zero for $120. If you come across a specific item that you have questions on, just post here and I'm sure someone can chime in.

If you go with a factory Aero upper, keep in mind that their M4E1 uppers are limited in handguard selection to the wide BAR format. They are a great setup and very simple to assemble, but somewhat limiting.