1. Not integrated. It will sit on the dash. You *might* be able to get the Bluetooth to hook to the car for speakers but not sure if the Avis Garmin models have BT.
2. Lots of cars have nav now (even non-premium cars like GMs, Ford's, etc).
3. Can't speak for the Charger but many 300s, Jeeps and Durangos (all Dodge family vehicles) have built in GPS.

If it were me,with a premium reservation, I'd walk the lot and find a few cars you like with nav that fit the premium category. Then approach the desk and ask for one of those cars. If they can't get you one with nav, then tack on a Garmin to the rental at the desk. No need to prepay as there's a good shot you can get it for free in the car.