I'm looking for a new motor, but the 2 dealers I have talked to did not have any 4 or 5 hp motors with a 25 inch shaft. Mercury and Yamaha both have to go to a 9.9 hp to get a 25 inch shaft.Are you looking for new or used motors? If new, any dealer would be glad to order one for you.
I have the same boat, a 1988 Hunter 18.5 and I am quite happy with my standard 20 in long shaft Mercury 4 stroke. I know Tohatsu, the maker of small Mercury and Nissan motors has come out with a 6 Hp motor especially for sailing. It has 25 inch shaft so that might interest you. Keep in mind the longer the shaft the more work it will be to tilt it out of the way while under sail and it will stick out more while maneuvering in tight quarters. I sail on the most insanely busiest Lake that has large powerboat wakes and I have little problem with motor cavitating. If you really feel you want the longer shaft, I would look into the Tohatsu sailmaster 6.
Good luck in your quest.
Bob
Tohatsu I believe is the oldest manufacturer of outboards now ever since OMC shut down . Not sure about British Seagull but if they are still in buisnesess, then Tohatsu would be the second oldest. They make generally good motors.
However, I would tend to thin k the 20 inch shaft is long enough based on the fact I was the top dealer with alot of experience on that boat who outsold that boat plus I introduced it for Hunter..