Going used is definately a feasible option. You have to look within your agency, the geographics, and the community to truly evaluate what you need. The emphasis is on need, not want. You said that you do not know what type, but that you do not want a van. Why not a type II? They are more economical, can fit in smaller spaces, and tend to provide a smoother ride.
What type of environment are you in?
Rural, urban?
What type of roads do you predominantly have?
Will you be transporting or providing first response?
Will you be providing rescue services or just EMS?
BLS / ALS?
What would your projected call volumes be?
Nearest backup resources, how far are they and what capabilities do they have?
What will your budget allow for acquisition, fuel, and operating costs?
Does your state regulatory agency have any restrictions in place that you need to be aware of?
Have you already applied for and received a license from your state to operate this unit?
There are many more questions that need to be answered, but these are some of the more basic ones..................
Research, research, and more research. You should arm yourself with this knowledge before spending 1 penny.