I think in reality a rust free Zed no longer exists or it never left the factory. A lot of metal was not properly treated to resist long term rust issues and the cars were often exposed to the harsh elements during transport on the ships or docks.
It would not be wise to think a used zed will be rust free. Mine was "Rust free" which means I will need to dig a little further to find it.
Also the definition of rust should also be mentioned. Surface rust or badly pitted to completly corroded away.
I find the biggest problem with these cars is the rust is most often in places you can't easily see until the car is fully stripped. The small tell tale bubbles under the paint are just the surface of the problem and the end result of years of corrosion.