I am no expert here. I am just someone who is vocal about people building their own, because it gives them a much better understanding of how the system operates. Building used to be MUCH cheaper, but lately the AR's have been flying off the shelves because of great pricing.
Quentin, for the record, is VERY knowledgeable about the platform and should definitely be consulted for the good information he possesses. He won't steer you wrong.
The lower receiver is one of the easier parts to buy. Pretty much any name brand made in the US is going to do you just fine. Billet is going to be much more expensive, and is not really required unless you want bragging rights at the range.
A good quality lower is pretty easy to come by, there are even vendors here that sell some great ones.
As long as you don't fine one priced at $49.99 on clearance from Bubba's Tire World and AR Center, you should be good to go.