Solo makes things pretty easy…
Start by finding a passenger van you like, you want one with lots of opening windows, and standing height if that’s important to you.
You want one that has decent factory floor, walls, and ceiling already, so all you have to do is remove the rear seats, then move your new camping interior in.
Many of us feel that you shouldn’t actually modify the van in any way, and for the most part I agree. If everything is kept portable or modular, you can rearrange your floor plan as needed. The original seat mounting points offer ample selections to secure everything, which is necessary.
Okay, so we need a bed, a kitchen, a bathroom, comfortable seating with a table/desk, heating, cooling, and power.
Your bed is going to be your biggest piece of furniture, so it’s placement should come first. Your choices are a regular bed, a futon style sofa/bed, or a dinette bed. Personally, I chose the futon sofa/bed option, and actually sleep on it in sofa mode unless I have company. I got a metal frame house style one, and merely extended the legs to make it high enough to clear the wheel well. This also allows a lot of storage underneath it. I got the frame for free off Craigslist, and bought camper pads for the cushions, and used cheap flannel sheet type summer sleeping bags to cover the foam. They zip up right around the cushions, and they’re easily washable. I placed mine lengthwise on the passenger side, running between the back doors and the side doors. In sofa position, this gives me an unobstructed aisle all the way from the front of the van to the back doors.
This is basically the layout I use: