Every seller or broker will tell you the engines are well maintained.
Any boatowner that "maintains his engines well" will have a maintenance log.
The log should detail the maintenance the engine and all other systems received during ownership.
Go over the log and compare it to the service requirements to assess the owners diligence.
I would not buy a boat without some sort of maintenance log or a folder full of service recipts.
I have 6,200 hours on my 1976 Lehman. We cruise at 1,700 rpm.
Bolt on parts; pumps, exhaust elbow, starter, injectors, transmission, hoses etc have been replaced or rebuilt over the last 20 years and 3,000 hours. The block, head and injection pump have not been touched, other than a valve stem seal replacement.
I'm thinking the injection pump should be rebuilt in the next couple years. Some smoke and oily sheen in the exhaust water.