Viper, welcome...you may regret it one day, as you're broke and still Jonesing for the latest bit of AR gear, but damn it's fun!
I agree absolutely with the advice to get the flat top, even if it means waiting. And, if you check around, I'm sure you can come up with the a flat top DPMS for even a little bit less. I've seen the Lo Pro at shows in this area for well under $600.
I also agree that the 16" ones are much more versatile than the 20", and unless you're varmint shooting at 400 yards, the velocity drop due to the shorter barrel isn't really a factor. And the 16" rifles are just as accurate, so you're not giving up anything there either.
My only experience with DPMS is with my .308LR...I love it, and really like the anodized coating DPMS uses. So far, I've had no problems with the rifle at all, and I would guess their other models are just as reliable and well made.
Consider a kit build, as well. It's much easier than you might think, and if you have access to a few tools, you really can do it yourself in an hour. All you really have to do is assemble the lower; the upper is already complete, and you simply have to mate it to the finished lower, and press the 2 pins in, then you head to the range.
You could build what you want for the same price, and be a little more flexible in terms of options...like float tubes, etc.
I know a place called Atlantic Arms in Maryland has Cav Arms finished 16" rifles for $599...plus whatever you're FFL dealer charges for the transfer...so you're in the $600 ballpark.
There's lots of options...don't rush into anything, look around...check out some shows. If you really want that rifle, I think $600 is a pretty good deal...you'd probably have to look pretty hard to find it for much under that price.