Start Simply
Make a list of every necessity in your daily life, down to the bone. A place to put clothes, a method to clean them. You won't need a large wardrobe. Focus on the things most critical to survival. Don't forget that this is about breaking free. That includes consumerism. Cut it down to the bare minimum, and as you live things will naturally accumulate. Starting with lots of junk is doing yourself a disservice.
Imagine
Now imagine yourself sitting in the van. How would you accomodate these necessities? Imagine yourself changing clothes, pulling a book off the bookshelf, washing a bowl. There are always some tasks that are easy and others that are extroardinarily difficult to do in a van. Pick your battles. If you're honest with yourself many things can be done elsewhere (showering at a gym). The more you build out your van the more you must compromise on everything.
Write
Now using some of those insights you have, put it on paper. Don't hesitate, don't overthink it. Write a list of everything you think your van should have. Then draw a picture. Build a mock bed out of cardboard. Rethink every single piece of the puzzle until you can't think of any other solutions. Stuck on a problem? You're probably overcomplicating it. Forget about cooking 3 course meals and refocus on the necessities.
Do!
Time and money are the two resources you'll be using. Don't squander them. Overthinking and overspending can be huge sinkholes if you let them. Most people never realize that they are only five or six steps from being on the road. Sleeping bag, clothes, trail mix, and water. Done. Remember the goal. To get away, cut expenses, and to be productive. Comfort, fancy gadgets, and other such trivials are not a part of that vision.