Unlike others here, I would recommend against a 16 year old m3 purchase.
How long could you possibly have been driving at this point? 1.5 years MAX?? Are you going DCT or manual?
I've been driving stick since I was 17, now I'm 27. Started with a GTI to an STI, to the M3. And even now it took me the better part of a day to get used to rear wheel drive with this 6mt.
If you do get it be CAREFUL, and for god's sake, dont showboat/brag to all your friends about your sick nasty driving skills with traction control off and end up wrapping your m3 and face around a tree, or another car.
In terms of the finance question, its pretty straight forward like the other guys said.
Also remember, just because you buy the car doesnt mean your done paying for things on it. Chips, dents, scratches (lets face it, your 16, most teens have an accident at some point, just hopefully not major), insurance,