With reference to the Mariner 50 on boatsandoutboards..
I sold one of those 50hp 3 cylinders Mariners last weekend for £1000, including a £60 remote fitting kit. Bill Hingham Marine quoted me a £1000 trade in value for my 2006, they are selling the 2006 model for £2200.
HOWEVER.. these are the short shaft version.. and would require hacking the transom on a searider down by 3 inches to make it fit. Seariders at best do not have much transom above the water when at rest.
DO NOT CUT YOUR TRANSOM DOWN... buy a longshaft.. Also those short shaft 50hp Mariners (actually are Tohatsu beneath the covers..) They wanted £800 + VAT to convert them to a long shaft.
I paid just £480 for a 4.5m avon of late, but refused to cut the transom down, for that exact reason..
I have had four 4 metre seariders, engined with 40hp yam 3 cylinder, 55 Yamaha 2 cylinder, 60 Mariner 3 cylinder, and a brand new 50hp 3 cylinder in 2006.
I would recommend Yamaha as the best brand stick with that choice if you can find one.
Generally I believe may be wrong but it is the short shaft engines you get on zap cats.
The avon is a tough little boat, can't beat them.
Advanced Power Boat Instructor