Slight correction: The two best days in a boater's life are when he buys his powerboat, and when he gets rid of it to buy a sailboat.
The rule with boats is, you are going to have problems with everything. An fresh-water cooled inboard is the best type of engine. A sterndrive (I/O)is the worst type of engine. The reason is that with a stern drive you have an inboard engine with an outboard drive. That's twice the complexity and twice the potential problems of an inboard. An outboard is a nice compromise, especially a four stroke.
Accessibility is the key, since you will either be doing most of the repair work yourself or someone will charge you a lot for labor, and the harder something is to get to, the more it will cost. Yamaha makes the best outboards, Mercury is next.
I have owned a stern drive boat and have friends that have also. I can't say this enough: unless you are on a lake and are buying the boat new or like new, stay away from them. They are nightmares. I wouldn't take one if you gave it to me, except to sink it for insurance money.
Of course the abosulute best engine is a diesel, but you'll not find one of those on a cheap runabout.
Good luck.
Bill Wallace