Does your Mom have good credit and make enough to cover entirely the lease should you not be able to pay a cent?
Banks are willing to lend too much usually. I'd be cautious about trying to get a new M if you've already been turned down under those conditions. Even if you could come up with a substantial increase to the downpayment, I think you'd be living too close to the edge.
An Audi RS isn't much less, is it?
2013 M3 coupe (Azurite Black, Black, Piano Black)
+ miata nut (2 miatas) + Mk6 GTI 5-door hatch (our mini-SUV