If the only problem you got are the miles, then 12K miles per year which is 1K miles a month. (10K or less is best because most leases go 10K miles a year but I'd say 12K is best because 1k miles per month is easier to work with instead of 8XX miles). If you ever get the e92 M3 and you still feel that the car has too much miles on it, avoid using it. However, you bought the car for you to drive, not for you to only look at.
Also consider asking when the e92 came out of the dealership so that you can count more accurately. It won't be fair for the car if it came out late Dec of 2010 or if it's retrieved late 2009.
I'm hoping to get an e90 M3 around 2013 (to replace my 328i) but knowing my mom, it's not happening...