So I see this armored SWB Sprinter cash-transit van once in a while...
 

Less than half the GVWR of a regular cash-transit van, about half the engine size, and due to its more compact layout it's also easier to maneuver in tight or crowded urban areas, which may decrease the overall time the engine is kept running, then some increase to fuel savings could also be expected...
https://blogger.googleusercontent.co...rink's.jpg

Maybe its aerodynamics are not so awful compared to an average cash-transit van too...
https://blogger.googleusercontent.co...ndicionado.jpg

My little brother used to wonder why you never see used armored cars for sale.

Usually, people who want an armored vehicle prefer something which wont stand out it's armored, eventually in order to dissuade theves who would otherwise be willing to attempt an assault presuming there are valuables inside. There might be some considerations regarding the legal aspects too, yet it may not be illegal to privately own a retired cash-transit van. But anyway, at least in Brazil some old cash-transit vans had been refurbished and donated to police corps.

I don't know why he cared, but some of them are bulletproofed with old phone books, so maybe they're well insulat[able]?

I know what you're thinking. Do it.

My office is next to a GARDA depot with maybe 100 trucks. Most of their trucks are built on medium duty truck platforms but they have started adding Ford Transits.

This article has a good description of why they are switching to smaller trucks:
https://www.fleetowner.com/operation...t-to-haul-cash

What I don't understand is they don't seem to have any brand loyalty with trucks from at least a half dozen manufacturers. That increases maintenance quite a bit.

Typical Garda Truck
https://blog.cvn.com/hubfs/Garda%20truck.jpg

I have seen some cash-transit vans built on Class-6 Mercedes-Benz trucks with that semi-bonnetted "round nose" AGL cowl still functional when I was a kid, yet they were more an exception than a rule, with most cash-transit vans here being based on a Class-5 frame and not using the stock truck cowl anymore, as they're now mostly built around a stripped chassis. I have already seen a few modern medium-duty and even heavy-duty cash-transit vans, yet they're now more often used for special cargo, including some refrigerated ones meant to carry high-cost medicines.