If you've spent most of your teenage years wrenching on carbureted small block Chevys, I/O's can be fun to work on. If you have a decent job and are not a "boating survivalist" (like the last owner apparently), I just assume I'll have to replace everything. I used to baby my 1990 Baja. I wanted to sell it but the shift bellows was starting to crack. Even though it didn't need it, I replaced the gimbal brng, bellows, u-joints, nylon lock nuts, thermostat, any questionable hoses, new EVERYTHING. I sold it in 24 hours, full asking price, and the new owner went boating that day trouble free.
Piece of mind out on the water is just something you can not put a price on. Sure, anything can happen, but where I boat, there is no towing service and mostly commercial traffic during the week. You're doing the right thing Motorcylepilot. You're gaining on years of lack of maintenance and you're gonna have a really nice rig in the end. Oh, and one more thing...never, ever beat on u-joints with a hammer. Even if you are taking them out!