There is a dubious generalization that RWC engines have a shorter lifespan than FWC engines, however, I don't think this is anything other than perception and opinion. What is a much more significant consideration is the maintenance history of the engine - ANY engine which is maintained properly is likely to last longer than a poorly maintained FWC engine. The better question you should concern yourself with when buying is how well the engine was maintained.
I agree that maintenance is key. However, that is unknowable here, unless the P.O. kept a maintenance log. I have a 35 year old Atomic 4 (RWC), rebuilt in 2005 and "treated" with a post-impeller, pre-block filter basket that in my mind keeps more crap out of the block that might cause issues. I also do a muriatic acid flush every four years or so, but I rarely find much in the way of corrosion.
To this I credit the fresh water of Lake Ontario and my 180F thermostat.
Seawater cooling is inherently trickier, of course, because you have to run the engine cooler than is ideal because of precipitating salts at 155 F (if I recall). An engine flush and a good look at the hose barbs will reveal much here.
As was noted, an issue with RWC engines used in salt water is scale buildup in the engine cooling passages. This can be dealt with by a periodic acid flush. It is also possible to add a freshwater cooling kit to the engine.