personally id never buy a used M, unless it is owned by a grandma which is not likely. i know exactly how i'd drive a new M if i got one, and im sure people think like me. too much $ for repairs and headaches w/ no warantee.
the only one i would do is an e30 325i and do a s50 swap then turbo it and put all m3 suspension on it.
then you'd have a 20k BEAST with a new engine. e30 ftw. well i probably wouldn't do that but it sounds fun. maybe one day.