Only fill the window holes themselves. I did this and you would never know it had windows. No need to do the full roof thing or even the whole sides thing. Cut the side supports out of another van to weld in where the windows were. I think you have all my pics of this process if not let me know and I will email them to you.

Same for the sunroof. Little trickier though as they will need to get the curving spot on.

The windows and the sunroof can be fabricated easy enough.

Rust panels for most other areas can be bought from Rare spares and welded in. This is usually cheaper than fabrication.

Why not do a full body swap with the other van you purchased if it is ok as far as rust goes?

If you need to get help, get many opinions as suggested and don't always go with the lowest quote. Ask if and how many of these things they have done before. Always go with someone who has experience not someone that says it should be easy enough.