When buying any car, but especially a BMW, just take it to a BMW dealer or independent and have them do a prepurchase inspection. (i'd rather go to a reputable BMW independent) - they'll give you an idea of what work it needs, and what condition it's in.
I'd never buy a used M car without one.
"When you were born, you cried and the world rejoiced. Live your life so that when you die, the world cries and you rejoice." - Cherokee Expression