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Fresh water cooling is the only way to go for long-term use in salt water, which will eat a block out. If you're sailing in fresh water and plan to stay there, not a big deal though.
A small diesel engine with proper maintenance should go something like 5000-10000 hours before needing an overhaul but if the 700 hours consist of all short runs under little load and no oil changes...Sure, it could need work. If you really want to come up to speed on diesel engines fast, contact Mack Boring and ask about their classes for boat owners.
But given "normal" use and reasonable oil changes (at least one per season) 700 hours shouldn't be a concern if the engine runs well (easy start, smooth operation, no smoke or oil on the water). There are Volvo engines that have been around forever, my only concern would be an older engine that might need parts, which Volvo apparently hand-crafts out of solid gold. ("Ka-ching")
One thing you might try to do is find a local diesel mechanic who comes well recommended by other boaters. Ask at the docks and see whose name comes up. Tell him you're looking to have an engine survey done when you buy your boat and ask what he'd charge, and don't be afraid to visit the shop or the mechanic to get a better impression of him.
When you don't know a lot about boats, you're right to be nervous about spending a lot of money. (More than what I keep in my pocket, is a lot of money to me.) A survey normally looks at the entire hull/boat, but a "rigging survey" and "engine survey" are usually done by separate people, a rigger and a mechanic, if you need/want them.
There's also more concern in recent years that standing rigging should absolutely be replaced at 20 years, even under the best of conditions. And, some would argue ten years "depending". With a 1990 boat, if the rigging is original, you'd probably want to replace the rigging either "real soon" or at least before taking it offshore. If you can haul a rag aloft on the rigging and it snags or catches on even one meathook--that means it is due for replacement.