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Nola, I pretty much have had THAT boat since 1985 here in South Flarida where I've used it to catch bass and snapper in shore, sailfish and dolphin offshore, ski and wake board in the canal in my backyard, dive in the Keys, and go out for sunset happy hours. My version has been a 1968 hull, which had wood console, wood seat (RPS), and wood decking. The 'glass console and seat are much better on maintenance. It's the perfect small boat! Especially for inland water. It'll go through shallows, get up on plane quick with the 90, trailer easy with a class one hitch and most any vehicle, and readily launch with one person. The only question here would be the condition. Once you satisfy yourself it's in good condition make an offer.
The boat works well for one, two, or up to about three people, after about four it starts to get crowded.
You won't see too many other boat brands on the water older than a old Boston Whaler. There are ways to detect if the hull is holding water but the probability is low and even lower that that is the reason the dude is selling it. I wouldn't worry to much about that.
Everything's a trade on a boat. So the boat's not in 'pretty condition'. And yeah, the hull pounds a little but that's mostly in open water when there are swells. That's about the only bad news. There's a lot of good news. If it's just choppy she'll get up on top and fly. Usually inland water is pretty smooth so that shouldn't be much of a concern. The hull is very stable and two-three-four guys can walk around in it without pitching somebody into the water.
The Yamaha 90 will make her go bout 40 mph and she'll get about 4 mpg, that's very good for any boat and even better with that kind of speed.
The two stroke 90 Yamaha is no longer available in the U.S. It's the best small engine ever made and is the best engine ever made for this Whaler. If the boat and engine are in good shape as is the trailer $5K is a good price and you can start with a lower offer and the guy will take it. The only thing it really 'needs' is a top for shade, and those aren't expensive. And you can get parts for Whalers all day long.
All that and if you go to sell it in five ten years it'll still be worth about the same!