Where to buy a campervan?

I have friends coming to France from Australia and they want to purchase a small second hand campervan. At the moment they are looking at a site in Holland. Which country is the best for them to purchase a van from? I know the prices in France are high for vehicles of any kind. They have seen a 1994 VW 1,9TD pop-top 2 berth camper for Eu4,500 on the Dutch site. Any advice you can give would be appreciated. They are planning to tour France for a year or so and will be house sitting for me for a month.

Thank you Marie-Claire Gauthier, apparently I should have posted this on the forums. I will tell them about the possibilities and definitely warn them about parking up at night.

The Dutch and the Belgians are really big into camper vans, so those are definitely good places, especially as they are usually careful with their vehicles. I think it's more and more difficult to find a spot to spend the night though...