Flip flop, try sleeping the vehicle for a few nights at home. Then you'll know what you need to be comfortable. Ditto with cooking. Try it at home first. Try some camping trips close to home.
Setting up a vehicle for long term travel is an iterative process. Set up a system for cooking, eating, sleeping, storing clothes, managing food, keeping warm or cool. try those systems out, and adjust if it is not quite right. You will likely change, adjust, fine tune some aspects of your set up many times. On you first trip You will bring some things that you don't end up using and need some things that you hadn't thought about.
Just do it! It doesn't have to be perfect, just good enough. And you will quickly learn what good enough is.
Another thought, what seems good enough once you start traveling will be different than what you thought you needed while planning from the enclosure of 4 walls and all those systems that run a house. In other words, priorities change on the road. What seemed essential in town becomes optional on the road.
All this is my perspective from my experience. Last year was my first year of traveling and I spent 4 months van dwelling before returning to sticks and bricks. This year I am one month out on this winters wanderings.
Some things changed and some didn't.
What changed? I have a different van, a whynter 12 volt cooler instead of buying ice for an ice chest, and better wiring on my solar system.
So again, JUST DO IT!
I am driving a different van this ear