no you dont have to be licensed unless you plan on transmitting on HAM Freq. what you need is a letter from you department comm. supervisor or the frequency licensee that gives you permission to transmit on their frequency in the specified area of operation that is on their FCC license. and you show that to who ever is going to program your radio. it can get complicated if your system uses a digital or truncated system so make sure you know what you are buying.
BTW, why do you need your own radio? isnt there one in the ambulance?