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.
I'm still working on my conversion (80% done) my word of advice: take a good look at Westfalia, it's based on the Fiat Ducato - Promaster here. http://www.westfalia-mobil.net/en/modelle/modelluebersicht/modelluebersicht.php
In MHO this is the most effective use of space. I did my drawings with a simple program called Omnigrafle, even if I have Autocad.
Make sure you have a well equipped shop with tools and enough space to handle plywood.
I got most of my supplies from Amazon.com, Home-Depot, RONA and IKEA.
But before starting I protected the whole vehicle with anti rust POR 15, inside and outside, plus a rubber coat underneath.
I purchased a composting toilet instead of a porta potty, I don't like using chemicals or disposing of them.
I also winterized the vehicle and my plumbing is all inside as I intend to use it occasionally in our crude winter (I'm in Gatineau).
Please feel free to contact me to share ideas, en français si vous voulez.
Oh, if we could just get our hands on some of that trim molding!!
Certainly, Westphalia has gotten it down to a science, but don't blindly fall into step until you have considered your personal needs/wants and other options described here and elsewhere. That's why we do our own.