Something to look into is how many people will have a smartphone. If not many (or for the ones who don't) I'd give them a cheap Android phone that does nothing but run your app. The phones can run on a group data plan (and like Republic Wireless' phones, WiFi whenever possible), and just be a more powerful kind of key fob that lets your members find, request, schedule, etc. cars. You could also use it as a key, requiring a password in the app to send a bluetooth signal that unlocks the car. I might also use these phones to connect to the cars' onboard computers whenever the car's in use, so that you can check fuel levels, engine status, etc. Some of that may be a bit further down the road -- you may want to go with more lowtech solutions while you get this up an running.

re: insurance and infrastructure, the best thing you can do is talk to people at ZipCar. You're not a threat to them, and they have the exact expertise you need.