Tractors that have simply been in continuous use for 30 years without 'refurbishing' should look about like my US Yanmars, below. If always parked indoors, maybe a little prettier.
But when you find a whole dealer lot of tractors with museum-quality paint, then obviously they aren't original.
There aren't any hard facts. One story is that the Yanmars in good condition are sold to US importers, these are the ones suitable to polish up and put right in service. They shouldn't need more than a new seat, probably tires, and maybe a muffler.
But the ones not good enough for this go to VN where they are run for another generation in the rice paddies - with no access to genuine Yanmar parts. Some are still good after this, some really need refurbishing. One poster here buys tractors in Japan and he says tractors going to VN today are simply poured into a container standing on end. The parts get cobbled onto the worn out tractors there, or used to build frankenstein 'Yanmars' that might consist of several models cobbled together. Of course that is the worst case, some of the VN rebuilts are simply good old tractors with nice paint. But there's no way to tell what you have until you get some hours on it and see how it turns out.
So its a gamble. A lot of good rebuilts, maybe not as nice as the paint suggests but still good value for the money. And then there's the occasional poster here who got the short end of the stick. Worst I read was the tractor bought sight unseen from a larger VN retailer. It took an hour to get it started to get it off the trailer when it arrived at his house. He soon measured very low compression and put substantial money into an overhaul - then immediately sold it. These stories aren't too common but they do appear here from time to time. (I won't bore you with similar stories I've read here - this one can stand for all of them).
Whatever you buy - go into it with your eyes wide open. Under all that pretty paint, it probably looked about like these two elderly Yanmars before the paint went on.