I think it's far to say that Disco II bodywork is much better protected against rust than Disco I, however, Disco II chassis do get rusty - usually only surface but definitely worth a look.
Disco II (assume you're after a TD5?) is a much more refined vehicle than Disco I, much more car-like to drive, slightly heavier on fuel, better higher-speed cruiser, excellent auto-box, unbelievable handling if fitted with ACE, straightforward to service.
I've had a number of TDi's and was wary of Disco II in part due to what I'd read on the forum pages, but honestly it's one of the best cars I've ever had. I often drive 5 hrs at a go without any discomfort.
Buy one and enjoy
Dave