I won't be much help, I'm only on my second season with my Merc 6hp. I ended up buying new after many months of looking. I found no horror stories for this motor. I was watching the ads and I was not seeing much of anything under 10hp in my area. Once in awhile a 20yr old war torn 2 stroke came up for top dollar.
My motor runs great, but its really too soon to swear by it as being great. I'll say I am very happy with it so far. The single cylinder 4 strokes do vibrate more than a nice old twin 2 cycle. I like the clean burning of the 4 stroke and not having to mess with oil.
I did experience an issue with hard starting if the fuel pump is completely dry. I don't use an external tank, so there is no easy way to prime the dry fuel pump. I posted a question here a few days ago, but no responses yet. After talking, thinking and googling deeper today, I think its 'normal' behavior. Here's the
whole story.
I normally use the motor several times a week with no issues. If I don't run the carb dry, it starts in 1 or 2 pulls. 4 pulls if I previously let the carb run out of gas.
If you don't use a motor much and drain the carb after use as well as use a stabilizer, it should be fine.