Good luck with the car.
A few things to warn you about. On all E46, the door seals droop in hot weather, window regulators and door lock actuators go bad pretty frequently.
Also on the SMG model M3s, the SMG solenoids go bad and cost a pretty penny. I hope you are getting a good warranty with it. Fixing small shit on that car is expensive. I wanted an M3 as well, but low mileage 04s were going for 40k out here.
Audio is a pain. Door speakers require some work to get any midbass out of them, and the trunk is sealed and the back seats are metal. So something that sounded good in your car now will have less output in the M.
SMGs are sweet to drive with the launch control and nice having the option of setting shift speeds for comfort and having Auto as well. Definiately a pleasure to drive.
Be sure to set aside money for repairs and tires.
Good luck with it.....sweet ride!
Also, check out e46fanatics.com lots of info.