You really do need to be very wary on this one. Is it a private seller or a dealer? Assuming the former, you have no idea where he is based, so cannot get any idea of whether this is his own caravan he is selling, or just one he intends to steal once he has a buyer!
At the very least, get the CRIS number from him and do an HPI check - this will cost about fifteen pounds, but will tell you if the person selling it is the registered owner, whether it has been reported stolen, has outstanding finance, etc, etc.
Before you hand over any money, it would be wise to do a very thorough inspection (including checking that the number you were given tallies with that etched on the windows). You should also arm yourself with a damp meter and check as thoroughly as you are able to ensure that there are no damp areas. Try to get receipts and other documents proving ownership and make sure there is a CRIS registration certificate.
My honest advice would be to leave this one alone, and wait until you find one that you can at least check properly before committing yourself. I know it may feel as if you will never find another one, but realistically, another will come up soon.