Yes normal vans like the GM and Ford are a bit cramped for height. Otherwise the bed layout should be setup so you can stretch out and be comfortable (hence long body vans). Storage space can be addressed in a few different ways, overhead, under bed, up the walls, use your imagination and make it safe (lips so stuff doesn't slide out).
Cargo hold? You mean something to keep cargo in place? Yup this is a must! Straps, etrack, cargo tie down rings etc. A 1500lb skid can slide forward in hard braking while only doing 35mph and can crush an amazing amount of stuff. I had one crush my bed from 72" to 18". I'm a bit over 18" wide (butt measurement) so I no longer fit the bed. Definitely carry a couple of decent outdoor folding chairs for those times you want to sit outside.
Heating is done with an Espar heater (I have a diesel engine) so heating and keeping engine warm are cured. Keeping cool well different story but many utilize a vent with a fan and you can get screens for the windows (different sizes). Lighting is done with low wattage stuff off a secondary battery that is isolated from the main starting ones but charged while rolling (or start engine to charge). Some of us utilize solar charged battery powered lighting. Again use your imagination. The sleeping area has to insulated or having heat is useless in the winter. Utilize lightweight materials and same for everything else. Keep your weight of stuff down as everything comes off your cargo capacity (weight wise and space).
Few utilize a generator and of course the obligatory power inverters for the fridges, tv, dvd player etc. By working out necessities and building properly you can have power, some toys (computer, games, movies), food storage, clothing storage and be able to haul over 1500lbs of cargo. Workout what power you need and what each device uses and build the system to handle accordingly.
Won't point at any companies but others can advise better on that point. Money wise well that's the fun part today.
Gas or diesel? I personally prefer a diesel for the better mileage and the supposed longer life. There are guys getting very high mileage out of the gassers so I suppose this is a personal and monetary choice. I love getting 600+ out of a tank on my diesel. Means fewer fuel stops while running.
Hope this helps a little.
Rob