I have found that my 84 Caravelle is beginning to show signs of
seam-rust... it was visually rust-freee when I bought it, but it has
been sitting around unused for 10 years...

I'm thinking that if it's worth having the rusty bits removed &
replaced, it might be worth going a little further. Having had a
5-door Hiace, it would be nice to install a driver's side (right)
slider. This would mean that I'd need the doorframe structure as
well. Window, lock & handles would be needed. It wouldn't matter what
year, so long as frame & door matched. What would these items be
worth?

The the thought happened... why not stretch it a little? Not a big
stretch, maybe about 90cm. Stretched through the sliders. Guess I'd
need extra frame sections & doorskins too.

Yeah, probably never happen.