Best vans to look for:
#1 Sprinter (if you have the money)
#2 Ford Econoline (adventure mobile if you can find one)
Don't waste your money on Westies. Their cult fallowing drives the prices up way higher than what they are worth. They are underpowered, expensive to repair, and require lots of maintenance.
Best way to power the van:
You going to want to buy somewhere close to 400 amp/hr AMG batteries. You can only run AGM batteries down 50% so effectively this will give you 200 amps of power. This will run you around 500$. Also you need a way to charge them. If you choose to go solar, a general rule of thumb is you need a 100 watt solar panel for each 100 amps of battery. You could also look at trying to charge from your alternator.
http://www.fridge-and-solar.net/dual_bat.htm. The ideal setup would probably be a combination of both.
To power a mini fridge, you first off need to get the right mini fridge. I would recommend going with a propane for 3 way fridge. If you go electric then you need to get one that draws very low amps. Theres a company that makes fridges meant to be powered off solar panels that might be worth looking into. Expect to spend lots of money here.
Heating the van:
This is a tough one. Catalytic heaters are safe but produce h20 as a by-product. This in turn makes everything wet and thus not ideal in very cold temps. You really need a vented Cat heater:
http://ventedcatheater.com/6.html. If you can get your hands on one (they always seem to be on back order by about 6 months) expect to pay a pretty penny.
Best way to further insulate the van:
Just buy thin insulation padding from home depot. Put it everywhere you can.
I could go on but I should be working. Expect to spend a shit-ton on whatever you get. I only use my van for spring/summer/fall camping for this reason. Winter is tough