The best answer here is that for recreational non sport fish service, hours will have little if anything to do with it. This is the most frequently asked question of boat purchases and is simply the wrong question. Marine engines die all the time, very very few ever have anything to do with being worn out, meaning worn rings and bearings and maybe valve train. The things you end up spending money in are simply not hours related. This is not a cat thing. All engines require maintenance.
The best used engine is the one that has a documented history of being used and maintained regularly. Emphasis on regularly. I'd happily take a 30 year old cat 3208, 325 hp or less or so, so long as you can show me maintenance history. The family I bought my boat from did not understand boats and would not let me have the maint logs, claiming they were lost. I frowned, but if actually told me what I needed to know. They freaked out when they realized they had a book with a pile of receipts in it. The layman thinks a history of having to spend money is bad. That's plain wrong. I want to see exactly that. The boat I passed up was the one with 500 hours and the salesman bragging about how little work had been done.
Now, I'm biased of course. I have 3208N with about 4500 hours. Later next week, I'll be starting to swap out oil coolers due to a gasket that is starting to weep. Nasty job, but that's what you do. You constantly interrogate and find what is acting up, and you go after it. If you ignore it, you lose an engine. Has nothing to do with hours. There is a lot more to an engine than rings and bearings.
You will spend many times the cost of renewing those just on a boats cooling system, fuel system and electrical than you will ever spend on an overhaul. Yet, because the engine makes a bit more fuss over the overhaul, that's all we pretend is important. Makes no sense. You might think about an overhaul once a decade or three if you are lucky. It's very infrequent. Now, a manifold, or a water pump or a heat exchanger. Heck, your doing something, somewhere all the time. Each and every year. That adds up. That's where your cash will go.
You want to know whether your old diesel is serviceable? Either you will be doing the work yourself, so how much do you need to teach yourself about this particular engine. That, or what is the support like for that particular engine IN YOUR LOCATION. That is the definition of an engines probable life.
We just need to stop asking about hours. It's just that the hours are there to look at. It's a human nature thing. Humans direct their attention at whatever is in front of them. If it's not there they ignore it. I build databases for a living, and it's the same thing there. If there is a measurement on a report, everybody wants to talk about it. Rarely do people stop to ask what isn't there. People make decisions in a vacuum all the time. That's the normal situation. With engines, there is only ever one usage based measure available, hours, so that's all anyone thinks about. Nobody even considers that if all they ever did was to measure the weight of the maintenance papers without even looking what was on them, they would have a higher correlation to lifespan than the engine hours themselves.
Hours are there to schedule maintenance. Nothing more. The hour meter has no knowledge of how those engines were run or how well they were maintained.