I can't speak to much about KBB or replacement costs, but the D2 has more room and is a bit wider (I think). I feel the D2 looks more balanced because of its added length, but I'm partial to the later '99-'03 models. I would imagine that over time the cost of replacement parts will line up just like the Series II-III parts do, of course, I haven't had to source those for a while..If I'm not wrong, you got your series just down the road from me in Maine, so if you're willing to travel even further, Colorado has a TON of Discos, (or even Range Rovers) all vintages, that would offer minimal rust and hopefully a decent price tag. I got a five year old D2 fully kitted with only 20,000 miles for almost nothing, and that was in '04, so there are people willing to part with great trucks for not too much money. FWIW.
1973 Series III, 109, Ex-Mod
1999 Discovery II, Cataclysmically Lost