The WRX and M3 are very nice cars, no doubt about it, but I have some real world advice for you mate. Maintenance will cost you a lot. The servicing and maintenance for my 300zx cost me a fortune* because parts do not lose value with cars. That M3 was not a cheap car 20 years ago and parts for beamers are notoriously dear. The WRX is a turbo'd performance car that is hugely popular with men in our age bracket. As others have said, prepare to walk straight into someone else's problem- wallet first.
The same can be said for insurance for us under 21 year old males. It is rightfully very expensive** because we do risky things and crash. Not saying you will, but I can nearly guarantee that one of your friends will by the time you're 20.
Not to mention the cost price of the cars you are looking at. My first car costed $1000. It served me very well for 2 years until I had a steady wage and some money behind me. Looking back, I'm very glad that's the way I went because I learnt many things owning that car. The importance of regular maintenance, wiping off bird droppings before it can damage paint, how a car handles when pushed and not letting other people drive your car if you aren't around. All of which I learnt the hard way and I'm glad I did so with something that wasn't worth $10k+.
I'll also point out that peer pressure is a very powerful thing. It's amazing what a person will do with a group of friends egging him on. Add to that a car with a very confidence inspiring driving feel and you could end up in a world of hurt.
Look, I can only tell you what I know from experience. Owning a sports car at a young age is great on a few levels. You'll love driving it, be very proud to call it your own and have tons of fun with it. But the financial reality is something of a nightmare. Buy yourself something a little cheaper, (a NA Impreza would be an excellent compromise) learn how to own and drive a car in the real world***, then after a year or two think about your options again. By then you will be a little bit wiser and may be able to afford an E46 M3 or WRX STI... Or you may have a wife, a leadfoot junior and a whole lot of commitments
*The 100k service for my 300zx was over $1700 and that's 'just' a Nissan. The thought of maintaining an M3 from the same era chills me to my core.
**My 86 is costing me $50 a week in insurance. I'm the only listed driver, living in the middle of no-where and using a locked garage. Running through your parents is a good idea but it'll still cost a pretty penny.
***With all due respect, autocross experience =/= driving experience.
EDIT: Read Hawkeye's thread too, chilling stuff.