Keep us posted, BTW. Best of luck. Vans do make the best campers (IMHO), even though I certainly like trucks with campers, shells, tear drop trailers, roof top tents on wagons, etc., etc....
Another option is a mini-van. Take a look at Jucy Rentals. We rented one, and the rear kitchen is pretty cool. However, we did not use the sink, and generally used the stove on a picnic table, but the cutting board, LED light, pull-out fridge and storage, was nice. Ours had a roof top tent that my daughter typically slept in. There was storage deep into the floor in the removed middle fold-down seats.
We saw a privately owned similar set-up - rear kitchen (removable it looked like) and a roof-top tent.
For a budget build, perhaps remove the rear seat, and have a read bed/sofa. Middle seats can fold into the floor and the kids can sleep there. Roof top box to get crap out of the van. The rear seat well could hold gear and/or a water tank. They get mid 20's for gas mileage. Anyways, just a thought. The important part is to get on the road. We did a month long trip with only a tent and two sub 5-year olds, out of a Ford Escort wagon.