I'm guessing this never made it to mediation/court? That sounds like a lot to be paying for child support, if I were you I'd tell her if she wants that much money out of me, I'll take the kid and she can pay.
Either way, not expecting you to divulge that type of info online, but I'm just playin' around anyway I agree here, and with what the others said, think of your income OP. Like I said, I make almost 60 a year and I'm not even thinking about a beemer...much less an M3. If you're making more than me, good for you, I'd like to hear what advice YOU have for ME ahahaha if not, just think about all the factors...buying a luxury car like that is a very, very expensive endeavour, and defaulting/missing payments on a 50k loan? That's not gonna' look good on the credit you need for that house you want.