Greetings & Welcome!
If you get a passenger van and just remove the rear seats, you’re both time and money ahead. Your floor, walls, and ceiling will already be finished, and you’ll have opening windows, so no roof vents will needed. Windows make a world of difference in the livability of your house.
A lot of people push for gutting vans and adding insulation, but I don’t. I made that mistake starting with a cargo van, and it was the most miserable camper I’ve ever had. Heating or cooling a window van is simple and easy and no added insulation is necessary. Thanks to having a window van and parking in the sun today, even though the high was only 49f, it was in the 70’s all day in my van and I didn’t need to run any heat. In my heavily insulated cargo van, I would have had to run heat all day long. Without covering my windows, and no added insulation, my window van costs less than half as much to heat than my smaller heavily insulated cargo van did. Solar heat is a real money saver in the winter.
My current camper van for the past 10 years or so was the cheapest and easiest of them all to build, and is head and heels better than all the rest. It started out life as a passenger shuttle van with the high top. Instead of gutting it, and building it, I just removed the rear seats and moved my new furniture in and secured it using the former seat mounting points. I made no modifications to the van itself, other than to add a battery isolator for my house battery. Not so much as a single screw hole, no holes in the roof, nothing. Easy to heat, easy to cool, plenty of power, and all with a view. It didn’t take insulation, solar panels, roof vents, expensive fridges, or anything built in at all. This portable build works really great because it can be used outside too if weather and location permit.
If my van does die beyond repair, I can easily swap my whole interior to a different van as well, saving me even more time, money, and headaches.