I was excited for this thread.
This is one of my favorite mental masturbation topics.
I was very disappointed at the paucity of mx creations, some of their shit has been pretty interesting, including homebrew .50 mgs.
For me, I don't want an ambulance or a dump truck or a cash in transit vehicle. All of them will be more ragged the fuck out than a ex-police car taxi cab.
The ideal vehicle base is a short bus school bus. Big fucking motor, heavy chassis with air brakes, lots of ground clearance, designed to be beat on. Most are not, because they are school buses.
(also, building one of these has to be situationally based, not making the horse for the cart. I would want to be able to maneuver on hard ball carrying a few of my friends from home fort to places to obtain... things and back in general civil unrest.)
Armoring: lots of videos on how to make really decent armor from melting down certain types of plastics. Also, don't use a welding blanket if you are going the fiberglas laminate route. Buy ballistic nylon, guess what it was initially designed for before people realized you could make long-wearing bags out of it?
for the windows, state department has roll on laminates for glass that will stop a crazy level of fragmenting and people trying to bust windows. That's what I would use. Makes things a little fuzzy to see, but you wouldn't have to waste a lot of time melting poly to match the window curvature.