What type (make / model) of boat is the engine in? This would help us determine what load the engine has been subjected to, what life it's lead.
To get between 10,000 - 20,000 hours, you really need to see under 25 HP per liter, and under 2000 RPM. I think the engine you are looking at runs at 46.6 HP/L @ 2500 RPM. This is not bad though, compared to modern engines where they run over 75 HP/L. approaching 2800-3000 RPM. Many High Output engines are trashed below 5000 hours, some a lot less than that, and they tend to suffer a lot of abuse too.
A photo is worth a thousand words. If we could see that, we could advise further. If the boat is well kept / clean / maintenance logs kept up, etc, then no way is the previous owner going to ignore the engines. This is good!
As suggested, you need to get an independent Engine Survey, with compression & oil analysis, etc carried out. It's not rocket science, a professional will know what the score is, no issues.