I would never keep a MW3 outside (not even a trad), in spite of a good cover. Dust will be a problem, and damp will be a problem. Probably you will not notice much the first months, but little by little you will see how the exposed situation will take it's toll. Anyway, it is the most fun car I ever have driven and I was on the verge of buying one. Well, I sobered up and got a trad instead. More practical, comfortable and easier to sell if/when the day comes. But as I said, the MW3 is hilarious fun. How it would have turned out after a year or two of ownership, I do not know.
