I bought my M3 Conv 4 months ago 4 12800 (a bit expensive) but i looked at over 10 and all of them had either modified bits stuck all over them or had been mis treated to some degree. I found my one had been used reguarly but loved to the max.Vanos noise,Rear diff leaking (very unusual),when the electric soft top is being put up watch the mechanism and strings as they are expensive to replace, make sure the aircon works (and gets really cold) after about 5mins if it cuts out or doesnt get cold enough their is a poss leak in the condenser or the pusher fan isn't working which costs around 180 upwards for a new one, to regas is around 40quid.Hard top is fantastic and saves the soft top in the winter, if you get a good one you will LOVE it, i still get excited driving to work in the morning. What a car!!!!!
Hope this helps