Yup, you need an AR, they're fun weapons to tinker with to be sure, and are accurate and have great ergos. In my experience you can go with just about any brand and do ok. Stag makes a ton of M4 copies, Bushmaster has alot more options and finally DPMS who makes a dizzying array of patterns in about as many calibers as you can handle.
Out of the rifles I have seen/shot/toyed with I like DPMS, seem to be a little more accurate out of the box on the whole. My 20" DPMS Hbar upper seems to print slightly better groups then my buddies RR Varmint A4. That being said, although you can have DPMS chrome a bbl in their "build your own rifle" store on their web site, it is rare to find one in a gun store with that option. So if that is important to you, you might consider Bushmaster. Although you could always put a new bbl later, these things are ridiculously easy to re barrel.
However every part in these rifles is pretty easy to change out/upgrade, I have about 7 different companies present in my AR build....
Which brings me to my next statement...
Consider rolling your own (which is what I did, scroll down to my post "My AR-15" for pics). AR's are easy to build and either screw, or are roll pinned together, you can buy it piece meal and assemble it as it comes in to spread out the cost and wait for sales on parts you need. The only component that needs to be shipped to an FFL is the stripped lower. If you go this route you will end up either saving a few bucks, or building a far better gun then you could have gotton buying one complete with the same amount of money. Not to mention just about all these companies out source their components to third parties anyways. (*Protip: the Spike's Tactical Lower matches EXACTLY the color of the DPMS uppers ;-))
I don't think I will ever buy a complete AR again... they are just so much fun to put together...