sounds like a "fish" story top me.
Meaning he knows what wrong with it, he's just giving you the sales version.
I'd get a marine surveyor to inspect it, and take his fee off the sale price.
Out drive for an old ALPH1 can be $600 used.
But you'll go back with something better, like a BRAVO and new its $2,500 and up.
I suspect the engine is tired after 12 years.
Boat detail is $300 to get a guy to clean it up.