Very tough question.
We try to cover a lot of that at the Maryland AR-15 Shooter Site at [URL]http://communities.msn.com/TheMarylandAR15ShootersSite&naventryid=100[/URL]
I like the 16" carbine the best, but you might consider the 20" standard as a good compromise first gun.
Learn to shoot the irons first, then look into optics.
If you want a red dot type sight, like the Reflex II or the Aimpoint, you don't have to have the flattop. These sights work well with forward mounts. A regular scope will be better ona flattop, but will work on a built-in carry handle model.
Muzzle brake - not needed. Get it if you like the looks. The difference is subtle on a 16" gun, I doubt I'd feel it on a 20"
Here in Maryland, a NIB Bushmaster is about $860. Some parts of the country are cheaper. Colts and Armalites are more.
I'd avoid a Frankengun (non-factory built) for your first AR. Get a good, reliable factory made gun. I'd suggest a Bushmaster for service, full milspec (as full as possible) and the chromelined bore. Colt, Armalite, DPMS, Olyarms are all valid choices as well. Don't ask folks here which is best (Bushmaster), you will get the Ford -vs- Chevy arguement (Chevy).