Insulate the van as if you are gonna live in it, because, well, you know. An Espar heater installed is roughly $1500. You can pay less, you can pay more, but that's a good ballpark. An aux battery, cables, battery isolator, maybe an inverter needs to be figured into that. Pretty in Illinois, check with Thermo King on prices. In Detroit, Espar is Michigan is who I'd recommend, since they install Espars in a lot of vans. (they're going to do some work on mine in the morning cuz mine quit working). The heater connects directly to the fuel tank. They have diesel and gasoline models.
I did my first winter without an Espar heater. I'll never do another one without it.
I'd recommend either very cheap used, or take a tour of 20 or 30 vans at truckstops. The problem is, you can get it all outfitted and then a few months later wish you'd done this or that differently, and it may be too late to redo it. That's why cheap for a trial and error vehicle. Or, get a good look at a many vans as possible, and find out what those drivers like and dislike about the way they did things,what they would do differently. When I first started I knew I wanted this, this and this, but after a few months I realized that I really wanted that, this and that. When I moved from the used Ford to the new Sprinter, I knew exactly what I needed to do with it.
I started off with a double height twin blow up air mattress and a sleeping bag. I had to replace the mattress every couple of months because it wore out after all those inflations and deflations, or splinters. In the Sprinter I have a permanent bunk so I can sleep while loaded, with a 6 inch foam mattress and a 3 inch memory foam topper.
You're going to be living in the van, and if it's not comfortable you won't want to stay out there in it.
