Bunch of questions, so let me know if I forget one.
Gas mileage - The Astro got around 22-25 unloaded and 13-15 loaded. The E350 got 8-15 unloaded and the same loaded. The Sprinter has gotten around 28-32 since purchased. MPG is better if you stay 55 or below, run 80 all day and it really sucks.
Fridge, he has one in the front section
Locks - Yes, they do get broken into and stolen. Yes it can cripple a business instantly if not prepared. We have shatterproof laminate on all of the glass and hardened deadbolts that work off a solenoid that locks into the chassis on all of our van doors
http://www.cargolock.com/products.html along with an alarm system and GPS. Some use magnetic locks, some use a nice big hasp on the doors with padlocks. Some use nothing special at all. I decided years ago NOT to have any signage on our vans and all of the windows tinted so you can not tell what we are when the van is parked. I think the signs on the van can be good advertising, but can also be very inviting to a criminal. Either way, insure the vehicle for full replacement cost of vehicle, inventory and equipment.
Buildout - I can build out a van in a day. The Sprinter spent the better part of a month at a cabinet shop. The rule of buying a new van is never sell your old one until your new one has been on the road a few months
Seating - I've seen everything from folding chairs to milk crates to wooden stools to boat seats and 5 gallon buckets in Locksmith vans. I use creeper seats personally and most that have Sprinters use rolling office chairs (they have more headroom). This is similar to mine
http://www.cumminstools.com/prodimg/3206.jpg
We dont allow hotlinked images
I'm impressed. 
