For what it's worth, my advice is a
on this, principally because you'd be putting yourself at financial risk to give in to seduction. Yes, a well maintained, NOT abused car will last a very long time, but the "M" designation cars are almost a guaranteed invitation to abuse. My bias, or sensibility, is that the stock BMWs offer more than ample power, and quite good fuel efficiency as well, at least on the highway, especially with a manual transmission. To me the "M"s are an advertisement for excess.
Some further things to consider... Do you have any indication of the service, and possibly the accident, history of this car? Have you figured in the insurance you'd be paying over and above what you're presently paying? Are you willing to pay more for the required premium gas on a car that probably gets significantly poorer gas mileage than your present car? Think all this out so the emotion doesn't get ahead of your head, this just seems too dicey for your present situation.