Mr. Rawles
I am a long time lurker on your site and would first like to thank you for all you do. I learn much from your site and finally read a topic I have some knowledge of. I operate a large ambulance service (75 units) and read the article about using ambulances as BOVs. I thought I might make a few observations.
It is true that the truck type ambulance have factory 4WD. However the majority of van type units have good aftermarket conversions. Most are done by Quimby. In fact I would only purchase a van type 4×4 from them. One down side to the truck type unit is that rescue squads are notorious for building a unit well above GVW. This causes all sorts of brake and suspension problems in the long term.
As for durability you may be surprised but the van type units have a longer service life as well as a lower cost of operation. They are usually lighter and have far more payload than the truck type. One big concern of a truck type ambulance is that the module is designed for remount. Now from a factory they are built well but at remount time all bets are off. They can truly be done by a shade tree mechanic and the electrical problems can be a nightmare. The van units will almost always come with the factory wiring and since they are all one unit the cabinets and structure seem to hold up better.
Excluding 4WD units, if I was getting one as a BOV, I would consider a van type Ford E350 built between 1990 and 1994 with the non-direct inject, non turbo engine. These units can easily go 400,000 plus miles. Consider keeping [one or more] glow plugs, a fuel pump, an extra set of injectors, and a crank position sensor as spare parts. These units are small, durable and easy-to-maneuver vehicles that handle well get acceptable mileage and are easy to obtain parts for.
One other thing to consider. How to paint the unit. In a true pre-TEOTWAWKI Get Out of Dodge situation having a vehicle that can appear similar to an emergency vehicle may not be a bad thing. With a van unit you could even have a magnetic sign with some sort of logo that could be added and removed at will. I can tell you an ambulance is rarely stopped or harassed. It is not unusual for them to go long distances and both LEOs and the public see out-of-area units all the time so it does not arouse a lot of suspicion. Of course you would have to check state and local laws.
Hope this gives some insight into ambulances. It is true they can often be found at low prices with low mileage and could make a great BOV, if selected carefully. – RB