First off the price is way too high, unless the car has few miles on it, but since you said the engine was rebuilt I'm going to assume that it doesn't. A 87 M6 retails for 10k with 98,000 miles on it. If you could get it for 8-9 I would grab it. Forget about putting a supercharger in it. The car will be a classic shortly (by definition) if not alreay (by enthuasist). If possible buy the car and DONT DRIVE IT. It it a classic one one of the original M series and it is doubtfull that there will be another M6... just like there will NEVER be another M1.