There are several things that make the Apple TV difficult to use in the car.
1. No touchscreen or integrated control (such as a powermate) support. This makes it difficult to operate in the car.
2. No DC-DC power supply. This means you need to operate it on an inverter.
3. No way to startup or shutdown automatically. See number 2, above.
4. No way to fully sleep device.
The Mac Mini with Front Row is very, very similar to the Apple TV and because it has the full OS X installed, is more flexible for the car environment. In addition, because it can run VM Ware or Parallels, it can run Windows based GPS programs or even full Windows front ends. Power up is quicker than minutes, although it still probably takes a good 60 seconds to get up and running.
The problems you are citing are not insurmountable. I know you can hack the Apple TV and you may or may not be able to solve many of the problems but for just $100 or $200 more you can find a used Mini that will make it much easier and will include Bluetooth.
The ATV is not completely unsuitable for the car, however. You can easily install it as a backseat entertainment system for passengers, although it doesn't come with a CD/DVD drive so all movies will need to be ripped and synched ahead of time.