I have owned a M Roadster for 18 years and I am only selling it because at my age I find it difficult to get into. I have seen many what appeared cosmetically good looking cars but under the surface a nightmare. Things to check, an accurate service history with copies of invoices from reputable BMW garages and even then take care. One main BMW agent in replacing the vanos stripped 9 of the cam cover bolts and cracked the cylinder head. Thousands of pounds of damage! Major things to watch for are vanos, rear boot welds, gearbox indents and suspension bushes. The are many sensors which can cause poor running so start the car from cold, gently warm the engine on a run then give it the beans. You will quickly notice if anything is wrong. Check underside for damage and corrosion. I prefer a higher mileage car that has been well maintained rather than a garage queen. They were built to be driven but need good maintenance using in preference OEM parts. Small dents and dings can easily be sorted at not a high cost but check panel fits for signs of major damage. These cars be tricky to drive especially in the wet. Again check all electrics I my opinion a weak point in BMWs. Roofs can be replaced again not at an extortionate cost
If you get a good one believe me it is fun with a capital F. I replaced mine with a Porsche 911 turbo and I have to admit if I could still fit in the M with the roof up I would not have changed even having experienced the new car. Good luck with your search.