Hi Richard
We have our caravan stored in the Languedoc - about half an hour away from Beziers. Some things come to mind - prices for second-hand French vans are
very high and French vans tend to be older and 'squarer' than a similarly priced model in this country. Take a look at this website for an idea:
http://www.leboncoin.fr/caravaning/offres/languedoc_roussillon/herault/?f=a&th=1&pe=9&q=Caravane. You also won't find the facilities you might find in a British van - probably no oven, perhaps no shower, no battery, less luxurious upholstery. Does the storage site buy and sell vans - if so they they might have a British owned van for sale.........!!? How will you view the van - presumably whilst you're down there, in which case what will you stay in whilst looking. I presume your storage site tows to the campsite for you - and brings it home again. Many of the regular French families on a site we use (Camping Neptune at Agde) have their caravans stored locally - many at Bessans Caravans - and on 'going-home-day' the storage site is back and forwards picking up and dropping off! I must admit it all looks very efficient!
There will be other issues once you've bought - French vans are registered in their own right - and have their own registration number. I'm not sure how this affects things like insurance, etc. We have British insurance, at about the same price we paid for insurance in England - and our storage costs are almost half for indoor storage than we paid here for outdoor storage! The other is servicing, and supply of spares, etc., though our storage site uses a British caravan engineer for their servicing (and we do our own!). I don't know how available spares are for older French caravans, judging from the state of some of them on site, people seem to 'make do and mend'!
It might be worth looking at alternative options, such as buying here, and getting a friendly British caravanner with a tow-car (such as us!!!!) to tow it down for you - providing all insurance is in place and expenses paid! I would think (from looking at several French vans in the past, having had a similar idea) that you'll get better value buying in this country.