Just to clear the air about MDS engines. When they first hit the market, their carb was poorly designed and even more poorly made. But the rest of the engine is fine. Guys would replace the carbs and get a great engine out of it. MDS has now replaced the carb, and the new ones run well. If you have an old MDS, you can send it back and get a new carb for it. (I don't know if you can just mail back the carb). There is a visual way to tell one carb from the other, based on the throttle arm, but I always forget which is which.

The other engines mentioned are all good ones as well.

I've only bought 1 engine off of ebay, and I got a great deal, but I've seen many auctions go way higher than the engine was worth, so you want to watch out and not pay too much. The 1/2 street price rule is a good one.