Greetings & Welcome!
I always try to keep the original interior if possible, or replace it with original from a wrecking yard.
If that’s not possible, cheap paneling is available at Home Depot, Lowes, etc. Whenever possible, I try to get woodgrain plastic, vinyl, or fiberglass (FRP) panels because they are water proof. They will easily bend to the curves in your van. I leave hidden gaps all the way around to accommodate air flow to prevent hidden moisture buildup. Add extra support behind anywhere you’ll be attaching cabinets etc. Adding diamond tuck & roll can also give you padded walls/ceiling too.
Since I’m an extreme weather camper, in temps on both ends of the scale, I don’t want any added insulation. The times you’d actually want it in a house works exactly the opposite in a vehicle. Colder in the winter, and an oven in the summer. Proper heating, cooling, and ventilation is a much better choice. If you have opening windows, I would also avoid leaky roof vents. $20 usb/12v fans & opening windows provide far better ventilation than $200+ roof vents ever will.
Floor & wheel well insulation is important though. Nothing beats the thick felt like insulation that comes in factory passenger vans. It’s not only great insulation, it is moisture wicking, fire proof/retarding, and sound deadener. With carpet on top, no moisture will ever get trapped in the floor. I put easily washable/replaceable throw rugs on top of my carpet. Carpet also means warmer floors in the winter, and cooler floors in the summer.
In the past, I used to “build” things in. Today, I keep everything modular, and just move them in and secure them, a much better and more flexible approach. I can easily change my layout as needs dictate, something you can’t easily do with built ins.
Good Luck & keep us posted!
"Happiness only comes before money in the dictionary." ~ Smilin Sam