My pair have 4700 and are doing well. If well cared for, 5500 hours should not be a concern, just a bargaining point. However, mine have been cared for.
The engine could be in wonderful internal condition, but if a cooling impeller, heat exchanger, or cooling hose fails while you are motoring along at 2300 rpm the engine could be severely and expensively damaged. Those 15,000 hour lifespans depend on the other stuff being replaced before it fails.
American Diesel at 804-435-3107 is a prime source for parts and information. Bob Smith is the owner and presents the Trawlerfest training seminar that Jeff mentions and that several on this forum have taken. He even answers questions over the phone, he's great to talk with.