I'm on a rural volunteer fire department and as a result, close to the EMS side of things, but have more experience/knowledge on the fire side. In my ~12 years, both fire/ems have gone through a lot of trucks.
Unless used frequently for transports, they often have high idle hours and low miles, or low in both, meaning they spent most of their lives just sitting. Neither mileage or hours are a good indicator of condition. Sitting is worse that driving after a certain age. Ambulances ESPECIALLY have an incredible amount of additional custom electronics, that can lead to hard to track down power draws and problems down the road as they age. As Reach said, they are HEAVY which cuts down on hauling and towing ability.
Unless bought new, and look up the title history to see, small departments themselves buy these trucks at government auctions from OTHER departments getting rid of them. What that means is you can either look at gov auctions yourself to eliminate a possible middleman, or know the other department is getting rid of the truck for a reason. Usually age, but sometimes condition.
The fire department I am part of is so small, that other than a recent built-to-order brush truck, our type 1/2 engines are 30 years old and are often found at auction for under $10k. For a full fire engine that holds 750 gallons, has working pumps and carries 4 fire fighters. They are not especially 'valuable' trucks.