Different courses have different standards for maximum db output, and also different degrees of enforcement. That question is best answered on a local forum, as the guys that drive your local track will know best what you can and can't get away with from an exhaust modification standpoint.
I would strongly recommend using track insurance, if you do choose to track an E9x M3. Your ultimate goal is to advance your driving skill and you'll be doing that by pushing yourself and your car to and past the limits. As you do that it is not enormously unlikely that you'll go off course and you may break a few things. Track insurance is a lifesaver here!
Of course, there is also some virtue in driving a cheaper, slower car as well. Learning with a car with a low power output can often improve your ability faster, as you quickly learn to really work the car in order to produce good results.