I am a retired paramedic, so I have more than enough experience riding on ambulances which ran the gamut from the old Cadillacs, Chevy Suburbans, vans, up to and including the big diesel units everyone is using now.
I would never buy a used ambulance or police car for a work vehicle, but that is just my opinion. US federal law mandates that an ambulance chassis must be replaced periodically because after 3-4 years on the road the vehicles are beat to death. The mechanics where I worked (Hillsborough County Fla) told me to multiply a vehicle's mileage by 3 to get a ballpark idea of the abuse the unit has received (if the ambulance has 100K miles on it, consider it to be mechanically equal to a passenger car with 300K miles on it). This comes from years of quick starts, driving balls to the wall with a cold engine, idling for 20-30 minutes at a time (with all lights on, a major drain to the electrical system), and jumping curbs, ditches etc (which can twist a chassis).
Expect your mileage to be below 10 MPG, and frequent repairs.
When I was shopping for a new van, every dealer I went so specifically asked if I was going to use the vehicle as an ambulance. I asked why everyone asked this question, and was told that the first thing ambulance builders do is overload the vehicle weight wise, which immediately voids the mfg. warranty.