Get some stuff from ebay and specifically here:
www.carbuilders.com.au/Just spent the last few weeks fitting out a 2019 Hiace as a camper - just like a house, your foundations are the key to a good fitout.
300W solar + 300 Ah LiFePO4 + 50L water.
Insulation is a must - you dont really need a vapour barrier, unless you want to camp above the snow line.
CarBuilders wont be "cheap", but also not too expensive (given the good results it achieves). *dont* use any fibreglass insulation - it is the wrong material for insulation for a lot of reasons. Use panel-deadener and some thick van-liner.
For cabinetry I use 6mm Marine Ply + some type of jointing material (such as 18mm pine) - cheap and easy to work with. A watered-down water based paint will seal the wood. Or you can go overboard and use marine-epoxy-paint.
Since you have wall-panels, you can just cover them in something nice/durable... anything will work, including plain old cotton-cloth with some spray glue.