What kind of van are you thinking about? Mini, E-series, sprinter? 4X4 or a soccer mom type. There are thousands of set ups, it really depends on what you are trying to do. City living or off the grid.
Get a really big van. Remove the wheels and pour a concrete slab for your van to sit on instead. Put up walls in your van to create "rooms"-- install a bathroom in one of the rooms. Install kitchen appliances in another room. Remove the roof of your van and install rafters, and then put a shingled roof on it. Replace all of the windows with actual windows. put horizontal siding on the side of your van.
I think an important thing to remember is that you don't really hang out in the van. It is mostly just for sleeping and storing your stuffs. You should be comfortable setting up camp where ever you find yourself. get yourself some nice camp chairs and a hammock or two.