The biggies in ambulances are Horton, Braun, and Wheeled Coach - Excellance is one of the smaller builders, but they have a solid reputation for building a good ambulance. Since you probably aren't going to use the ambulance as an ambulance, the more important question is the vehicle chassis. The miles sound about right for a small town ambulance service, but check the maintenance records to see what has been done. If you buy it, I suggest changing all the fluids and doing a lube job. Also, plan on new brake lines fairly soon. Since ambulances are washed regularly, they tend to sit over a long trench drain in a fire station, and the high humidity tends to kill brake lines in about 10 years or so. That same advice also applies to collector fire trucks as well - they have the same problem unless they have air brakes.
The only caution could be if the ambulance you're looking at uses digital switching. The VMUX (Weldon Mfg) system is used by many builders, so you can track down parts fairly easily (not cheap, though - a new node will run about $500), but if they use a proprietary system (like Horton does), you may be forced to go to them for any help or parts. I suggest that you verify that the controls all work and do what they need to do before you buy the ambulance. Also, remember that you may need unique cables and software if you want to change the system's programming.
In my own case, my ambulance uses conventional relays and diodes in the control system. It's a little more cumbersome, but all I need are standard automotive relays at $10 or so each. I can change "programming" with some wire and a few diodes.
