I built mine. Already had an AR15, so needed an upper and a mag block. For the upper, I got an Anderson lightweight with shell deflector and no forward assist. Barrel, I got the Faxon 10.5" (registered SBR). I had a Nordic free float tube laying around, and a Colt 9mm flash hider. I bought a Palmetto bolt, mags, and mag block on the classified on AR15.com. I also added the Rock River Arms 9mm mag catch and buffer, and 13 quarters in the buffer tube to keep the bolt from over travelling. Bought a used C-more here, and done.
If you already have an AR, build another upper for it. Add a mag block and buffer. But, nothing wrong with buying another gun if you want it. Palmetto stuff is good, but never in stock. Even when it's on sale, it isn't available. And if it is in stock, it'll be a month before it ships. And as far as ordering straight from RRA, I've had a 9mm upper on order for 2 months, still haven't heard from them. And building it is fun. Check the rimfire/pistol forum on AR15.com, lots of great info.