Are you thinking risers and exhaust manifolds? I just did the heat exchanger, oil cooler and transmission cooler on my Lehman for $1k in parts. Easy job to do yourself.
Just to share info----modern Yanmars ( my most recent research) are all turbocharged. Likewise Cummins, Volvo. I Wish they still made Lehman's.
I just did extensive pricing with several Certified Yanmar Service Yards, Mack Boring in NJ, in Niemic Ma, Niantic CT, Rewport RI and Precision in westchester NY.
A 22 yr old boat with 4 LHA STP engine that has never had any cooling maintenance done, regardless of how many hours would need the 3 Heat exchangers REPLACED, the turbo intercooler REPLACED due to corrosion.
Parts are 8K , labor is 4K. I could do it myself , but still incur 4 K in those parts, not to mention another several thou for replacement hoses, shaft dripless seal etc. If the turbo was bad that would be anouther 3500 to buy and 1500 to install.
Those are hard numbers. A 6 CYL is more.
As I said, please forgive me , I don't know the extent of your diesel experience, so I thought Id share mine. I've looked at nearly a dozen boats in the past 5 months, not one had kept up with the manufacturers service intervals overall, and especially not for the Cooling system, a few had done a rid lyme flush AFTER the engine overheated, but none did the proper recommended service. 'So my point is --- be careful before you buy--- ask for receipt for all maintenance, don't rely on word of mouth, or just to talk to the yard that did the work. a blown 4 cal diesel is abut 35K to buy new and 5K labor to replace.
I'm looking for a boat fr long distance cruising round trip 150 miles round trip, thats safe and reliable. If I had bought any of those boats ( nearly double the price they were 18 months ago) I would have to had put in another 25 K or more to make them safe & reliable.
If you just go off the dock weekends for short picnics--- you have less risk, when you cruise longer distances, you need a well maintained boat .
The bigger the diesel the more expensive to service. and I read that Volvo's parts pricing are off the deep end............Yanmar's pricing are getting there.