OP I think the big thing is to do your homework. The problem with ARs is that, unless you know what's what, it's easy for pretenders to hide as contenders. DPMS and Bushmaster are bottom feeders but often cost just as much as a good maker.
Just so we are on the same page, I'm not saying you need to spend a boatload to get a good gun. I'm saying that if you do your homework you can get a good gun for the same amount of money that they were asking for the DPMS.
I'd start with a complete lower from PSA (as I posted above). I've seen them twice this month on sale for $199.99 +$15 shipping to your FFL. They are on sale right now!
Complete PSA AR15 Lower - Classic Edition - No Magazine
You don't have an FFL? Read this:
http://www.defensivecarry.com/forum/ffl-dealer-discussion/130352-how-find-local-ffl.html
FFLs should only charge about $20 or $25. If someone wants to charge more than $25 then find someone else. FFLs are a dime a dozen so find someone who is easy to work with and doesn't charge too much.
Once you have a decent lower you can shop any of the big names and get your upper. PSA sells a boat load of them and uppers get shipped to your door, not to your FFL. Just remember to wait for a sale. Also, many uppers are sold without a charging handle and BCG. Make sure you read up before you buy one.
All you need to do to instal an upper on a lower is sit it on top and push in two retaining pins. It's easier to do than field stripping a Glock.
Lastly you can buy a whole rifle. This rifle is on sale and comes with a $400 optic on top and a $150 free float rail. <--- You cannot beat that deal!
PSA 16'' PA-15 HF SS12G2 Carbine
If you don't like the Aimpoint you can sell it for about $400. That leaves you with a nice AR15 for
$679.99 + $15 shipping and +$20 for the FFL.
Get some!