Engines and mechanical parts do have a recommended ‘design life’ and often the 5,000-8,000hr figure is used. I’ve heard of fishing vessels that have main and auxiliary engines exceeding 20,000 hours without major maintenance issues. I have also heard of a John Deere engine that was regularly serviced that had over 30,000 hrs on the clock and I’m certain there will be more examples of engines lasting longer.
Overloading or having poor maintenance practises will radically reduce an engine's service life, and as you can see, there is no simple rule when buying a boat. You need to carefully consider all the aspects mentioned in this article, trust your gut feeling about the owner's honesty, and look after your engines - they, in turn, will look after you.