Like Giok I could drive mine at the national speed limit, it still returned 35+ mpg (diesel) and felt safe in all but the worst crosswinds.

Part of the trick IMO is to keep all possible weight low down, and to buy a van which is as little modified from the base vehicle as possible, thus retaining the structural integrity and handling characteristics. Avoid high roof additions which increase drag and susceptibility to side force from wind or other vehicles (I soon got to find I preferred to cook sitting down, where is it written that you have to stand?).

Above all avoid the van conversions in which an oversize, caravan-like body has been dropped onto an under-powered and under-sized chassis; they look as stable as a camel wearing high heels, and their progress is about as effective. Luckily these seem to have gone out of fashion.

Of course if money is plentiful then the A-class, built from the bare chassis could well be the right route, but consider where you intend to use it as alpine roads, or Devon lanes become more of a challenge as vehicle size increases.