That aluminum 4.0 liter V8 dates back to 1961 as a Buick motor. Rover made some adjustments to it over the years, but it retains much of its basic heritage. Back in the Sixties, coolants were not up to dealing with aluminum components, and cooling system failures were common. With today's modern mixtures, of course, that's not a problem.
Discos have never been much for resale value -- something they owe more to their spotty reliability than anything else. They're certainly capable off road, but I would not be rushing out to buy one since I don't relish frequent mechanical and electrical failure.