I would say build one.
You can build them how you want, not pay retail + markup and have a bunch of parts you never wanted in the first place.
Lower receivers are plentiful. Don't pay 200+ for one. Call a few shops. They still have them. If they didn't, there no way I could have ended up with 6 in the past 2 months. There was a day that I could have bought 15 of them...but what for?
Lower parts kits are harder to find but you can still find them. You'll pay 70-85 for them rather than 60-65 but I've taken dumps that cost more than the price difference.
When I look for parts, I can find them. It may be harder to find than before but they are still there and you can still get good deals.
As far as upper recievers or full uppers are concerned, I will be going the used route and picking up some on AR15.com's equipment exchange. People are always selling uppers there.
My lowers were $130, parts kit were 70. That's $200 + $100 for a stock.
Then all I need is the upper which will cost me about 450 to 600. If I want to go fancy I can spend more if I want.
I think if I wanted an AR and didn't care so much about the components I could build one for about 800 right now.