I agree, this would be better in the M-16 or legal forum. First off decide what you want, how much you want to spend. An M-16 is the most versatile, dedicated .22lr upper, 9mm upper, all sorts of variations can be made on the AR platform. Learn as much as possible about the process of getting into class three. Lots of laws and forms to familiarize yourself with. Also talk to as many people as possible, there are a number of tips that will make the process go smoothly. Check out
www.subguns.com for some good info. Good luck.
Add: I have learned a substantial amount about class three in the past year. One concessus I have experienced is don't waste your time with burst weapons. Especially with an M-16. It is very easy to control two and three shot bursts when on full auto as well as being able to dump that 30 rounder in a few seconds and then trying to wipe that damn grin off your face.