Guest, I guess if you *are* going to buy an old car, then a tank is a good move. :-)
To be fair, there are quite a few older 4x4s around. But they were always built with heavier metalwork, which would naturally take the rustworms longer to get through. Bodywork was usually the killer on old cars (although engines do wear out too - I bet you've had at least one new engine and gearbox over that time?).
The rustworm problem is why there's precisely one old car that I'd buy - Reliant Scimitar. Doesn't look too bad, in a 70s sort of way, and the fibreglass body is immune to rust. Shame Reliant self-destructed by deciding to make invalid carriages instead of proper cars, really.