Greetings & Welcome!
What a cute van!
I would start by removing that bulkhead, and replacing the original bench seat with super comfortable swivel, reclining bucket seats. These can provide seating, and with a table, it’s also a dining/work area. If it has the rubber floor mats, scrap those and convert it to carpet with appropriate padding, so it’s not trapping moisture underneath causing your floors to rot out.
Next I would add rain guards to your front windows and create your ventilation system. You DO NOT want roof vents, or to put holes in your roof for any reason. I would build a mailbox ventilator that attaches to the inside of a cracked passenger window, and blows air backwards into the cabin, then exits out a cracked drivers side window. (I can provide directions for these if needed.)
Yoga/puzzle type rubber mats glued to the wall side of the interior paneling/ceiling you’ll be installing, preserving the air space & air flow to the outside skin to prevent moisture build up. Thick felt type carpet mat, and carpeting for the floor and wheel wells. (This wicks the moisture away from the metal floor and to the interior where it can be evaporated out.) Add a washable throw rug on top.
For the cabin, the VW Vanagon Westy designs are hard to beat. Just add your toilet/shower, house battery with an isolator, heating & cooling, and you’re ready for just about anything. Put sliding doors on all your cabinets, rather than swinging doors, so they don’t require interior space to open them, and they’ll stay closed while traveling without the need of extra catches.
I wouldn’t build my stove or sinks in, just a bare counter top, that you can sit a camp stove on, or dishpans for sinks. The advantage is that both can be used outdoors as well without duplication. I just use trigger spray bottles for water for my dish pan sinks. Very handy. I intentionally don’t have any plumbing that can freeze.
The smaller the space, the harder it is to have a full featured camper, but it can be done.
"Those who believe money can't buy hapiness, don't have either." ~ An Anonymous Vandweller