I will be selling my M for R540k with 42k on the clock and motoplan till april 2013. Its an m-dynamic model with carbon roof and i would not even consider a non m-dynamic. Im not trying to sell it here, i already hv a possible buyer, just to give you a club reference.
The m-dynamic option allows control of steering, dampers and throttle response and makes all the difference. When its off, the car is really dull to drive.
Things to check for would be comments, general condition, tyre life (new they cost about 6k each) , motorplan optional extras such as navigation, lighting package, carbon vs sunroof etc.
While 2 cars may look the same at first view, each of them can have very different optional extras.
Good luck, there are some pearls out there