As it was stored in a barn it's possible the person didn't know or forgot it was a leaker. This rot however doesn't fit the descrbed condition of 'nice condition' in any way, shape or form and should not be your problem.
The courts don't like it if you have made no attempt to redress issues so you need to contact the seller and politely point out that the top and sides of the van and rotten, and that you'd like your money back please.
The fact you have had to inspect the damage by cutting the interior is IMO irrelevant because it's already rotten, and any remedial work needs the damage to be assessed.
Good luck, but also you need to stop 'owning' this problem and politely and firmly reject it as 'not as advertised', because no judge in the land is going to describe a van rotten on the top and sides as 'nice condition'. If you need to go to small claims BTW it's pretty quick, cheap and easy these days, but first of all try being polite, notify them of the problems and ask for your money back.
Essentially you bought it on the basis of it being in 'nice condition', but it isn't.
Thank you..... I'm hopeful that my be the way forward. I've sent the seller a letter, it will be a nervous few days hoping for a response.
Cheers
Matt