I tend to make a distinction between the RRC/90/110/Defender/D1 heritage and the Dii (not to mention P38). There are some similarities between the D1 and Dii; but quite a few substantive differences-the porosity of the block, for example got much worse by the time you started seeing Bosch engine management in the Dii's. You really don't see the HG and slipped liner issues nearly as much on the early V8's; at least I haven't. Frame thicknesss is different too of course.Personal opinion on why Disco I & II's and RRC's are cheap. They both have the same heritage, a chassis and drivetrain derived from the Defender.
A great reference: http://www.robisonservice.com/servicedep/buying-a-land-rover.asp