I personally would not do it only because of the reasons you have given .
You could remove the radiator hose from the bottom and flush it with tap water , then with distilled water. Add Distilled water to the Pre-stone coolant .
You are better of selling your car privately at a retail cost . The dealer is going to offer you a whole sale price because he will later sell you car or a profit . Yes, any dealer should trade of any car as the dealer who much he would offer you then you could add another $1000 to it and sell...