If it doesn't eat oil... doesn't belch blue smoke and the radiator doesn't emit lots of bubbles at high rpm (errh... don't open the cap when it's hot, though...)... it should be relatively okay. Maybe get an emissions certificate.

I don't know the going price for secondhand Starexes... I know it should be somewhere between 350-450k... I'd try to get them to discount it on the knowledge that it was overhauled. But the fact that they told you it was is probably a sign of good faith.