Being a mobile service engineer, i would strongly recommend not buying a cheap damp meter as they are not industry recognised and will give you a false reading, we have to use the protimeter damp tester, which retail at £140 and upwards depending which model you go for, but as stated in the previous threads these can be hired from any hire place, they will give you a demonstration on how they work and are very simple to use.
I would also recommend you try to get a mobile service engineer if you are serious in buying it, to check over the van as they will do checks on gas, electrics, 12v chassis, wheels, tyres delamination of floors and a damp test. If you decide not to get an engineer my advice would be to visually check everything ask to see appliances working, the biggest mistake is the tyres, which could be an expensive job to replace after you bought a van, people dont realise that the industry recommends them replaced after 5 years never more than 7 there is a date stamp on the sidewall of the tyre for example 427 which would translate to the 42nd week of the year 2007. Sorry if you already know this just dont like to see people ripped off, vans are expensive enough without having to put things right.
Hope this helps Pete