When buying an AR, the barrel and upper receiver are the most important parts. The rest is easy to change.
I opted for a 14.5" bbl with permanently attached compensator for my first. Soon I was dissapointed to learn I couldn't easily put a free float forearm on, since the compensator would have to be cut off, and an even LONGER one added afterwards. This was upsetting.
I have since sold that rifle. I would recommend a 16" for your first. After that, get others.
As far as TYPE of barrel, I recommend a M4 profile, because it is lighter than the HBAR, and works just as well for 99% of what you'll do with it. If you wanted an A1 "pencil-barrel", I think that would also be just fine. Careful, some manufacturers (Rock River) sell HBARs as their standard barrel. Be sure it's skinny under the handguards, because otherwise they look the same. Usually, if there is a notch cut out on the front of the barrel, it means it's skinny under the handguards (M4 Profile.)
I would suggest a flattop (A3) upper receiver unless you are CERTAIN you want the permanent carry handle (A2.) I have an A2 20" rifle, that I SPECIFICALLY wanted as an M16A2 clone for my US Military rifle collection. Otherwise I'd always get the flattop version. I have a 16" A3 with removeable carry handle as my M4 clone in my collection.
Bushmaster is great (I have three), but I'd suggest looking at Stag Arms, or at least buy the upper receiver (complete with bolt) off the internet (Eagle Firearms, or a "plethora" of others in the industry forum), then buy the lower receiver separately also off the internet in order to save some money (excise tax AND sales tax.)
Like this from Eagle Firearms:
Complete Upper: $469
and then the complete lower: $235
Total: $704 (+ $25 for the FFL transfer?)