For the opinions thing,
The doors: When clicked, the door opens and your player mounts in that seat. But if someone is already in that seat the door opens anyways, but you won't mount.
Damage system: Visual damage would be cool, but having steering and power loses would be annoying with our current physics as cars are always hitting things they shouldn't. ("hitting" the road when going up a ramp of plates).
Police lights: I would say light key, because this way you can turn the lights on and off in 3rd person without having to switch into first person.