short answer for FCC licensing is:
* private for-profit ambulance service: Apply for frequencies under "business/industrial" pool.
* private for-profit ambulance service, dispatched under a E911 contract: you would be eligible to apply for frequencies under "public safety pool", but I would stay away from public safety freqs, more of a hassle. Just get "business/industrial" frequencies, and you can get VHF or UHF, and if you want direct/simplex or a repeater pair.
WRBK832
here is our FCC license, as an example, which includes a for-profit ambulance service like yours:
We exclusively use VHF which has worked out the best for us. So it includes some VHF repeaters and VHF direct/simplex tactical frequencies. For radios we use a mixture of Kenwood TK-5710H which are VHF 110-watts high-powered mobiles, some Kenwood NX-5700 VHF 50-watts, and Motorola XPR5550e VHF 50-watt mobiles. The Kenwoods we use analog & P25 modes and the Motorola is analog & DMR (i.e. Motorola MotoTRBO) digital. With the digital VHF repeaters we get about 50-80 miles of coverage. VHF works best if you are out in the country, and not in a city.