I had an M6 cabrio from a year old, the depreciation for the first owner was savage and the depreciation for me was merely eye watering was losing a grand a month, add that to a grand a month fuel costs. My average mpg was 14 and if you're not driving it like that you might as well have a 630i.
It is a heavy car so tyres are a major expense too along with the brakes.
Personally I wouldn't do it. I had m cars for 3 years and warranty claims were £20,000 in that time. If you can afford to lose £15k then do it but a better bet would be leasing a new one. There are some cracking deals available.