i've owned a couple of very high k cars (VK w/ 700,000)... if the donk's been reco'd, then it shouldn;t be a problem. If not, it may still be prefectly useable, if a little (very) tired.
Your main problem will be the body. Up around the 500,000 k mark, every bolt on the car has worked loose, every piece of trim will be squeaky, the suspension will most likely be shagged, the diff will be noisy, gearboxes will be loose...
The type of use is also important. A rep who drives Brisbane-Canberra will rack up a lot of k's, but if the car is treated well, it'll still be better than, say, my mother's work hack AU that's driven and thrashed by every *individual* around. It's done 43,000 and it is ROOTED!
But then again, it all depends... My ZH has had 400,000k's of loving use by one owner (daily drive of 30k's for 27 years, + regular trips to Sydney), and it feels fine. It's always been garaged and has a full service history, including yearly transmission/radiator flushes, even the odd diff oil change. The 351 has been reconditioned, as have the FMX gearbox and the shockies. The car feels solid, still looks shit-hot and is great to drive.
My EF has 240K ks on it and I have no problems at all. Starts first go,m has plenty of power and is relativley economical. It was originally a Far Nth Qld car so did all of it's ks on country roads. A car that had done Ks like that in the city would probably feel a lot looser. The Ks dont matter as much as how it's been treated during those Ks