hi Steve,
It would help if you could tell us the make, model, and age of the caravans you are intending to look at as you mentioned both caravans I assume you have a couple in mind, this would help as maybe some of the forum members who have owned the particular vans may be able to help further with what to look out for.
in general terms however Martins post gives a good starting point I would only add (and without being too negative) explain that at the price quoted while it is feasable to get a very good van it is unlikley that it would get you a van less than 10/12 years old maybe even 14 and a this age there will be undoubtedly be a fair amout of wear and tear and in many cases vans of this age are sold as seen.
this does not mean that all vans will be riddled with damp or any such malady but it is unlikly a warranty would be offered.
a good dealer should however if pressed supply a damp certificate and gas and electricity safety certificate this time of year is a good time to buy though as with the cold damp weather sales will be slow and all the new models for 2012 will be arriving soon which mean dealers will be trying to clear out some space with a bit of arm twisting it is supprising just how much can be squeesed out them to get a sale, above all if your not sure or suspect something walk away. as the dealer up the road will be more amenable.
colin.