Russf-- I think your broker is being somewhat optimistic in his desire to make a sale but I don't believe he is totally out of line. We've had our boat for eleven years, and it was 25 years old when we bought it. As far as the core engines themselves are concerned, all we have done to them in the last eleven years is change the oil and filters, change the belts, change the coolant, and replace the zinc annodes. We've had the valves adjusted once. This is all routine maintenance following the service intervals spelled out in the manual.
But.... stuff wears out or fails and there is no "timetable" for when these things will occur. So over the same eleven years when the core engines needed only routine maintenance as described by your broker, we also had to do the following:
Replace the coolant pump on one engine. Replace flexible impellers in the raw water pumps. Replace the oil and transmission fluid heat exchangers. Replace the exhaust systems aft of the manifolds on both engines. Replace all eight engine mounts. Replace an injection pipe that developed a pinhole leak. Rebuild both alternators. Replace both raw water cooling pumps with new and more reliable pumps (this was a precautionary item, not something done as a result of a failure).
I don't believe Sunchaser is saying that fluid, belt changes, and a valve adjustment will cost several thousand dollars, although at today's labor rates it could come close, but that lax maintenance in the past could mean that other items like heat exhangers, hoses, pumps, etc. may be nearing the end of their service lives and need overhauling or replacement to get you to the point where routine maintenance is all you have to do (for awhile).
As a single data point that might be helpful, the cost to have our local diesel shop change the oil and filters in our engines used to be about $600. This was for two Baldwin oil filters, eight fuel filters of different sizes, six gallons of Delo 400 30wt oil, and the labor. I have been doing our own servicing for many years now so the only dollar cost is for the filters and oil. This is still close to a couple hundred bucks but at least we don't incur a labor rate of close to $100 an hour.