Its all about the body. Check for rust or rust repair in the roof, wheel arches and windscreen frame.
Its had the fuel filter unit replaced with an aftermarket type, but that's not a problem.
The air filter pipe has that duct tape around it, if its been like that for a long time in dusty conditions and leaking unfiltered air in, it can be a problem. Its cheap to fix(the pipe).
The coolant system appears to have some leakage on the top hose, internal corrosion is not a common problem with a 1HZ, unless its really been let go.
How are those speakers mounted on the roof? Im not a fan of drilling through the roof. Its a rust magnet
Now 500000klms is a lot, even for a 1HZ. Sure, there are 1 million klm units out there , but they are the exception. Most 1HZ get tired after 400000. The ones that do the high miles are usually buses that never cool down and come back to a depot with a dedicated workshop servicing them at the correct intervals.
Landcruisers that are 20 years old have usually had 3-4 owners. It only takes one to ignore the servicing and all the other owner's good work no longer counts.
I would want to see a cold start and I mean seeing it start after it has not run for 12 hours or more. Tired diesels usually smoke grey and often run on 4-5 cyls until warmed up. The more tired the are, the longer it takes for all cyls to start firing.
A good 1HZ should started with 3 seconds of cranking and quick flash of white smoke when cold.
With 500000, I would expect the gearbox to be close to be needing a rebuild
Dont go into it with rose coloured glasses, but I have to admit its nice and original and someone appears to have taken care of the body and interior. Maybe they have cared for the mechanicals as well.
It could be a genuine bargain, it certainly looks better than some recent monstrosities for sale.
Take a pic of that aluminium plate on the left hand side of the firewall. It has useful info that we can decode.