I do plan on insulating. Not going to go overboard on it, but would love to be able to get up some reflextix with the 1/2 inch space from metal(bubble wrap if that suffices as air gap), then an inch or so of foam or polyiso board(would like it if the insulation was flat with the ribs), then a layer of plywood, then the wall covering(some form of paneling material). Not sure if I need a vapor barrier under paneling, but if recommended, that would also be there. Also not sure if plywood is needed, but I am planning on small shelves/cupboards where it won't affect head space. Now that I know I am going to commit to a cargo van, I will have to do more than light reading on the "how to" of it.
As to window size, they will have to fit between ribs. Possibly 12x12 towards rear on both sides. Circular seems like it would be spiffy, but I also want to be able to open them...so prob not those.