The biggest damage is what you cant see, but you already know that. Its something I would tackle. I am a mechanic who has paint and staighten a few cars etc in my day.
On pricing
Search the net, ebay, car sales in caravan section and get a feel for the price of a good condition van of simular age and model.
Work out what you think it may cost to repair and add 50%
Take the sexond figure from the fisrt and subtract another 25% off that
Then bid you max knowing them figures
best of luck, every man needs a project
cheers
blaze