First welcome aboard!
How many hours is a varaible, was it used in fresh or salt water, open or closed cooling, maintained properly???
Assuming salt water this engine should last 1000 - 2000 hours. At 560 it is most likely time to replace the exhaust manifolds and elbows as these are water cooled for operating in confined space. You are looking at ~$500 for aftermarket and ~ $800 for mercruiser parts.
Also the raw water impeller (rubber) has ~ 2year life to avoid issues (can last longer but at $50 forthe parts not worth the gamble IMO).If you have an Alpha outdrive it's located in the middle of the unit, if you have a bravo it's mounted on the lower starboard side of the engine (right facing forward).
Depending on how faded it may need a light sanding wit 1000 to 2000 grit paper, rubbing compond, polishing compond, wax. Start at the end and work your way back, i.e. wax if not good enough the polish, ...
Regarding valve of the boat try ebay or yachtworld.com
Good luck and keep us posted your adventure, hopefully all fair seas