I had a 1999 Yar-craft 1678bt and the hull/ride is as good as you can get in a boat that size. I would easily take that boat out in rough water and it was better than many 17-18 foot aluminum console boats I’ve been in. Tracks/trolls extremely well. I also fish a lot of skinny rivers/lakes and it shines there. A very versatile boat. If I get too old to launch a bigger boat someday, I’d have no problem going back to that boat or similar.
The only thing I would recommend that you do is take it to a reputable boat repair shop and have them give it a work out before you buy. I took mine to a shop where the guy jumped up on the motor and tried to swing it back/forth to see if the transom was soft at all. He removed transducer plates and stuck a screwdriver in the holes, in the floor too, checking for any wood rot. The older glass hulls themselves are good, but transoms and floors are wood before about 2005. Glass over wood on the transom is fine until water gets in there and doesn’t dry out, because it’s sealed in there – it will rot. Repairing a transom on a boat like that is $4-6k and a floor at least half that amount. Not worth buying it…
I looked at plenty of boats until I bought that 1678. It was the most solid boat by far. I walked away from several others.
Once I knew it was solid, I took the 50hp off and replaced with a new 60hp 4 stroke (the max for that hull). The 50hp did about 24-25 mph but had a slow hole shot. The 60 popped out of the hole and topped out around 30-31. Tillers are notoriously very conservatively rated, so I think you really need to consider the max rating, especially if you fish with 2-3 people.
Great boats, I remain a big fan of Yar-craft hulls…