Yes, Honda 4 stroke oil is available. Now the 9.9 and 15 are the same engine. Yes, that should be a nice eng. Are present 9.9 is a limited 15hp. In the low hours it is best to vary the running speed.
Salt water is a problem with everything.
But proper preventative attendance will keep the engine in good shape. Prices vary greatly. Shop around is my best advice. Our outboards are really rather simple. The the Carb versions are basic in computer control.