Completely different platforms.
Leaf springs vs coil springs be the #1 difference.
LHD is not hard to find at all, in either 80 or 70. Usually the 70’s are much more used up though. For most of the world, a 70 is a work truck and an 80 would be a family hauler or luxury.
Educate yourself before purchase. Get a 3rd party to inspect if you can’t and don’t k ow what you are looking at. Use a reputable shop for this, preferably one familiar with both models and diesels.
Plenty of people get burned buying these trucks because they are not educated in them. Just because somebody talks a bunch or seems solid doesn’t mean they are. Remember used car salesmen are not to be trusted.
Cheers