Tracking is fun. To go one or two times with the e90 won't hurt too much, but like others have said you'll get addicted and then it will hurt if you continue with the M...lots. If you leave MDM on the car is very forgiving and unless you are really being foolish you won't get into too much trouble in the beginner class.
It is a great way to learn what you can and cannot do in your car which is why I think it's a good idea to use that at least for now. You'll begin to understand how much grip your tires can maintain before you get into trouble which will translate over to the real world. Knowing how much your tires can take will help make you a better, more confident driver all around. If you get a specific track car, while great, you don't drive that on the road every day and it won't necessarily translate as well to your DD e90.