I almost commited to doing same thing this past fall with regular van body and may still do it. Although I would rather have a shuttle bus the ambulances have the heavy duty suspension and power to handle the water. They also have air systems that may be sufficient at keeping any fume levels down on the inside. I would just paint them solid white with maybe a blue stripe pattern and convert the red lights to spot/flood lights.
In the vans it is usually a one ton suspension. When you look at a rig like that it is probably just like a regular dually delivery truck. If you keep your eyes open there are some ambulances that are a full rear air bag system. Boy those ride real nice!!!!