Good maintenance+good usage
I think "good usage" is just as important as good maintenance. For example, even a diesel that is maintained by the book will not last long if it is only run infrequently, for short intervals, or without paying attention to it. On the other hand, a diesel that is run frequently, run long enough to get up to its working temperature (and to keep it there long enough to burn off moisture), by an attentive operator will last a long time. A fifteen year old diesel with only 300 hours on it may not have been run enough. My guess is that one that age with 1000 hours on it could be in the same or better shape, because it has been run longer/more frequently. If you're buying a boat, have the engine checked out, too, preferably by someone familiar with them. Good luck.Randy