Damp is a problem - or possibly several problems!
First you have to identify the source, it may be some way from the internal signs
Then you have to seal it, not always easy when vans are on the road, vibrating & wobbling.
Finally you have to deal with the internal consequences which may not be merely decorative. Wooden frames can rot, mould & fungus can grow hidden behind panels. You may need to do quite a bit of reconstruction & redecoration.
So, if there is endemic or widespread damp & mouldy smells, just run a mile. On the other hand, if it is clear where the leak is & it is very recent with no long term damage, then maybe, just maybe, it will be fine. The judgement is yours.
EDIT: I wouldn't even think of paying for it to be done professionally, I suspect the cost could exceed the value of the van!