Hiring a van to take to France can be very expensive, once you say you want to take it abroard the price really rises. Ferry prices vary greatly depending on which crossings you want to take but using google you can find the cheapest prices quite quickly.
Make sure you all get an european health insurance card.
Always have european breakdown cover, make sure you have all your driving documents to hand. Check what you need to have in your van/car for driving in France such as Fluoro jackets, spare bulb sets, warning Triangles etc.
I'm sure some of the lake owners on here will offer a bit of advice later.