Before doing anything, I'd recommend speaking with the dealer who sold you the van to find out what the manufacturer’s corrosion protection is. The van might already have an underseal and adding something to the underbody might cause a reaction that will damage it or trap moisture. Any wax or oil-based underseal should protect the underbody, but get the dealer to provide a list of products that are compatible with what's already there. For the exterior, I'd use Autoglym High Definition Wax.