Welcome. There are lots of build threads here with plans that have worked out. I would suggest you plan lots and use CAD. Thats Cardboard Aided Design, get a few refrigerator or furniture boxes and mock up what you think inside the van and then ponder it a bit. Insulate with a product that won’t absorb condensation to about R-10 and decide if you want a campervan or a class B. The campervan might use a portable pottie, jugs for water and drain, portable stove and built in sink and refrigerator. A class B would have a compartment bathroom/shower, tanks under the floor, permanent kitchen and storage. Both would have a bed. It is now easy to have either without propane which can be a hazard by utilizing solar, batteries, and an inverter.
If your van is a 136 it will work better as a campervan. A 159 or Ext. can easily be a class B.