If you can find an idiot to pay you $9000 for a 91'M5 with a bad motor hurry up and do the deal before he blinks - because anyone with an ounce of sense would not give you that kind of money for that car -especially with all of the tacky aftermarket crap you did to it(the playstation is very juvenile). I purchaed a 1991 M5 in mint condition 9 months ago as a 1 owner, 105,000 documented miles, no mods, dealer only service and only paid $14,000! Which was still over Kelly Blue Book - and we all know Kelly Blue Book is very high compared to real world prices - There are tons of people that try to sell M cars for a mint - the never ever sell - If you had a 91' M5 with 132,000 miles (average for a 91')if it is in excellent condition and you were selling to an individual high book would be about $11,900 (a lot less for a dealer trade). A new motor will not increase the value of your car beyond high book plus a low mileage add. You will probally be better off fixing your current motor, then put the car back to stock and get what you can out of it.