Aside from the condition of the motor you should look for rot in the
transom and stringer/deck system. Any problems there might cost more to
have fixed than you pay for the boat. Walk around inside and check for
soft spots in the deck. Remove any deck access plates and use a mirror
and flashlight to see what you can under the deck. A built in gas tank
might be a problem after 15+ years, look for gas in the bilge or the
smell of gas under the deck. BTW, diesel, kerosene or bad gasoline will
burn on a stick but it won't run a gas engine. Tap around with your
knuckles or a wrench or something on the transom and deck, solid will
sound solid, soft(rot) will sound different.