If you base all the body work ( rust repairs, dent removal, panel replacement ect ) including stripping it down and fitting back up on a $35 an hour cost ( not including paint prep, painting and consumables ) you should be able to work out what the body work cost you. Break the job down into panel sized areas to work out the amount of hours ( 8 hours a door, 8 hours a guard ect. roof, side panels and larger areas need to be doubled in the amount of hours ). This may seem excessive if you have good guards or doors but the hours you save on them will easily be chewed up if you have to spend longer on other areas. If you spent 8 hours a day 5 days a week working on your van you should be able to knock it over in 4 to 5 weeks. Add another 2 weeks for paint and another week for fitting it back up.
If you can buy guards or doors ect that are in good to excellent condition and only require stripping and a few small dents fixing for under $300 it is better to to do this than spend 10 or more hours trying to fix rusted out or severely bent ones. I have spent too many hours than I care to remember repairing shit panels just because the owner wants to "keep it original".