I have to admitt, I'm no fan of air cooled engines. I'd go for a water
cooled two stroke engine. The two stroke is simpler and lighter than a
four stroke, and it cost less as well. The bad part, you have to add
oil to the gas, you'll get over it, trust me. As far as storage, just
sit the engine upright for a short period of time before putting it
away, to let the water drain out. People store them on their side all
the time. I would go with a Mercury, lots of dealers, no problem
getting it fixed if it has a problem. And if you put some hours on them
they all wear (Don't pay much attention to sailboat guy's who swear by
some brands, but don't realize that they put next to zero hours on
their engines. They all last forever if you never use them.) Figure on
an engine like this being with you for a long long time. I would stay
away from a Japanese engine for that reason. I don't care what anyone
says, your just not going to be able to get parts for them years from
now. They'll just tell you to buy a new one.

I'm sure this might get a few people going, but I grew up on the water,
I got my first small outboard 38 years ago when I was 10 years old. I
put lots of hours on them over the years, it's fun from time to time to
just putt around in a little boat. I'm not new to this. I've seen
people have to throw out engines just because they couldn't buy some
silly part anymore, it can, and does, happen.