I couldn't be without a memory foam topper... or 12! Thickness, 2 inches at least but the thicker they are, the more difficult they are to roll up and store. I've got a full Tempur mattress on my bed at home and have toppers for every other bed in the house and varying sizes for the van, depending if I want to just sleep on the sofas, pull out half of the 7ft bed or the whole kaboodle. I haven't found much difference between cheap or expensive ones but I always buy a 'Coolmax' cover if the topper doesn't come with one.
Tip: If you make up your van bed every day, buy a couple of those click-release suitcase luggage straps to anchor the topper into a smaller, tidy roll for stowing... or just use a couple of old 'slidey' type belts. Bonus is the straps act as handles for lifting etc