I have bought several used motors and I am not scared of buying used. I am very skeptical when I go look at one to purchase however! I check compression with all plugs removed, I also look at the plugs to see how they have been running, I look for wear and tear, I like a motor that appears to have been taken care of, I always want to hear it running and really like to take it for a ride before i commit, the bottom of the motor will tell you if it has been used on a river and had the snot beat out of the lower unit(doesn't always mean mechanical failure). I have a 93 15hp and a 88 25hp, both are Johnson and both are very good shape for their year. My brother had an 85 Johnson 15hp that I used for several years. My dad has a 68 15hp Johnson I used to use before I bought my own boat(why buy one when I can use one), the darn thing runs like a top but looks like crap!!! So you can't judge a book by it's cover in all bookshelves...
I'm not saying Johnson is what you should buy, it's just what I have used. LOL When I look for a motor I tend to look for something in the 80's and late 90's as I believe those motors were built a lot better then some of newer models. As do most of the folks on boating and outboard motor forums.
I don't own or have ever owned a 4-stroke outboard so I wouldn't know the pro's and con's between them. I do know with the crappy gas we have now-days, I use sea-foam and marine ethanol treatment in every tank with little extra oil!
If your anywhere close to Kokomo you are welcome to try my 15hp to see how you would like a 15hp motor.