I would get a compression test done and have all the drive fluids checked. The first thing they are going to tell you is they changed the stator fluid when doing the impeller, but there is also the pinion gear oil to check as well. Find out how many hours are on the engine and try and get it out for a water test.
Normally having it checked by a dealer would be enough, but you can never trust the dealer when they are the one selling it. If they are willing, try and find a near by certified Mercury repair shop to have an independent check done. Other than that just crawl all over the boat and make sure everything works, including all lights the bilge pump and all accessories.
Aaron:cheers:
