Actually raw water cooled inboard marine cooling systems are pretty simple (especially older VP engines) not like what Merc has on their modern engines that have the easy drain system, I have never seen so many hoses and plastic fittings as are there are on my brother's 2020 4.5. It works fine for now but....salt water never forgets....
If it overheats there are causes that need to be tracked down, I'd say the seller has to fix that, if it has been an issue or else lower the price to compensate for this issue, however, past damage because of chronic overheating can be expensive to repair (leaking cyl head gaskets, cracked cyl heads, etc)
Best thing you can do is a thorough water test including running it on plane for at least 10 min. Monitor temps, should be between 150-160, after coming off plane it may rise to 170-175 for a few min but then come back down....
Things to check:
Impeller, housing and wear plate
water transfer tube and rubber grommets in the outdrive
thermostat and housing for clogging
manifolds and elbows for clogging
that's about it.
I have a similar system on my old OMC Cobra, most times when it ran hot was due to sucking up sand, which wears the impeller and wear plate. As long as I replace the impeller on schedule and the exhaust system (every 5-7 in my salt water environment), I have not had chronic overheating issues.
