I have a 15HP Mercury that hasn,t been run in 10years. The motor was new 15yrs ago. Someone told me it would probably need a new impeller. True, or unlikely? If so, what would this be costwise. I'm going to sell the boat and motor, a 16' flat bottom, and I'm wondering what to ask for it. I'm located right near the Mississippi river in NW Illinois. Thanks alot.