Very tough subject here. you have alot of options to look at.
First off I will start with the swap:
If you intend on doing this yourself you better have some serious help from friends who know a SHITLOAD about cars. Swaps are very intense if you don't know to much and don't have the proper equipment. You could be looking at a full month of work (considering you will be keeping your life while you do this) to fully complete everything once all the little miscellaneous bits and pieces are finished. The plus side like you said is that you would have cheaper insurance and you would learn a ton about cars.
BUT, if you think about a swap you need to address a few other parts of the car at the same time. You will definately need to upgrade your brakes. Stock brakes will keel over and die within minutes of abuse from an S50 engine. They would have no chance of survival.
Also, your suspension will still be that of a stock 325is. Not tuned like an M coupe.
So, the money you will save from insurance and swapping yourself just might go bye bye when you buy performance brakes and new suspension to bring your ride up to M status.
Mcoupe:
Do I really have to say the pro's of buying an M coupe???? Great car, can't really say anything bad about it.
Except...more money, higher insurance and two seats (I think)?....
Another option that you could look into:
Driving school. Unless you want the power of the S50 or Mcoupe for straight line accel. you could get the same results from your car by learning how to push it to the fullest. Now, I do not know your driving skill at this time, and I am not trying to say anything here. But most likely you are not able to get the most out of your current car in the twisties or on a track. That being said, you probably would not be much faster in a swapped car or in an M coupe. (see spiffs vids. where he owns people with stock SAAB....car doesn't always matter)
If it were me I would wait at least until i was 18 to do either a swap or purchase an M. Your insurace should really drop when you become a legal adult (assuming you have a clean record). Then your options might seem a little easier to decide upon.
Hope this helped