For daysailing/coastal, though, an Atomic 4 makes sense, if it's been maintained. For long motor-sailing, the diesel wins. I've rebuilt an Atomic 4, and am preparing to rebuild a diesel. Both have their pluses, but if you're going to run the engine an hour or less per trip, stick with gas. In this price/size range, a repower with diesel could easily exceed the price of the entire boat.
No one objects to an under-27 footer having a 9.9 or smaller long-shaft for an engine, but somehow there's still the idea that a gas inboard makes no sense. If you just use it to get in and out of the mooring field and go head to wind, a diesel makes no sense. The reason I may have to rebuild mine is closely related to lack of use: diesels want to be turned on and left on, whereas a low-compression gas engine isn't harmed by intermittent, brief use.