I quoted a cat long block last year at 12k. If I were buying, I’d plan on zeroing the hours of the water pumps, heat exchanger and HE end caps, the exhaust risers and all belts/hoses, also do a quick valve adjust. You could well have a perfectly running engine and no survey deficiencies, but this would still be my minimal budget. Should run roughly $5k per engine and set them up to be happy for a long time.
If they had higher hours or slobbering, I might consider doing heads, but even cat recommends a lot more hours before that is slated, you will have to own the boat a long time to get there.
Lots of people won’t do any of this until it breaks, I like to have the machinery in a known state and this is what it would roughly cost to do so. I would not ignore the heat exchanger, at this age metal end caps are often going to be close to failure.