Hi, don't know if i can add too much to this already.
But I would wait to find an original van in original paint. even if the paint is thin and there is light surface rust it'll be fine. all the best panels that i have got for my van were completely original (sandblast, etch, re dolly tiny imperfections, prime) oh so quick to fix.

I would NOT buy a van if there was rust in the gutters or the body seam where the van meets ute bit (this is scary expensive rust). These areas are quite complicated and very time consuming to repair correctly. If you do repair these areas the inside needs as much attention as the outer skin as you don't want it to rust out again.

if you are not fussed with model, go WB (in my opinion) as they will be cheaper and are "newer" therefore less time to rust/deteriorate. It is also quite cheap to find a standard HJ-HZ front end.

If you are dead keen for a windowless van buy a windowless van, don't attempt to patch a windowed van. I have never seen one in the flesh that has been done properly.
I'm assuming to do one properly you would have to spend a lot of time or money to get a good result.

I purchased a "rust free" van as an uncompleted project which the previous owner had spent 10k plus in body work.... In the end the van was completely bare metaled to start all over again.

I have been through 6 cars in total now to get the right parts (very cheaply, but have spent so much time) and countless extra hanging panels.

If i were to do it again i would save like mad and spend the money on a really nice original van then start saving for the repaint. In the end it will be so much cheaper.

All of this info is only based on my experiences. people i would love to hear some feedback on my ranting.