Dano - If you have not already checked the transon here are a few ideas:
1. on the trailer with the boat secured, trim the motor down and stand on the cavitation plate and bounce up/down and look for any flex in the transom.
2. Also on the trailer, back out a screw in the transom (for a transducer or such) and look for water comming out when you pull the screw and if the screw is rusted. Just have a good tube of silicone to reseal the screw when you put it back in. I've been told if water comes out especialy if its rusty/dark it means there is water in the transom into the wood in the transom which will over time cause flex and problems.
3. on the water, if you haven't already, check all the switches, pumps and electronics, including the bildge. If its water tight, find a way to get water into the bildge (can do it on the trailer at home with a hose) to make sure the bildge pump works.
4. If you can have a mechanic check the motor out and compression. Also look at the trailer, check the back of the wheels to see if there is grease all over the back, if so the seal is blown and will need a new seal and possible new races and bearings.
5. If it runs good, looks good, fits your cash budget - buy it!!