20k is too much.
Agree bottom paint is a large negative. Sitting in water, possibly brackish is not good on hull, electronics and likely engine. Examine how much pitting is on bronze running gear to get some idea.
Regardless, going back to gel is not likely cost effective or going to yield a good result as the new gel is just a thick paint layer not like the original gel. Bottom paint that does not stay wet will not have as long a life and will require a hard paint if trailered regularly which is more expensive. Budget an extra 800-1200 every 3 years max for bottom paint.
Exhaust manifolds and risers would be a concern with corrosion. That will be another $900ish plus labor.
IF he is the first owner and does not know hour meter, another red flag.
Assume no trailer?
Agree a full inspection is definitely in order. More so this time than an average purchase.
NADA has a low of 23k and average of 26k so I would feel better at 16-17k knowing I am in for a full interior and its prior history if the inspection goes well.
Down even more if new interior means ALL vinyl and not just seat skins.
It did buff out nice but I would consider this more a "project" boat so be prepared.
1991 MariStar 240sc, 454, tower, WetSounds, Krypt and Rockford Fosgate