Car ignition lock + arduino

step down is overkill, a voltage divider, or even just a 100K series resistor would be enough. (if you disable the internal pullup)

Step down is a good way to power the arduino.

What are you really trying to do? If you just need to be on when the car is on, power the circuit from the appropriate rail. If you need to be always on but know when the car is on, I would power the thing from the battery (there should be at least one fused circuit for always-on accessories) and use a protection circuit of some sort running into a pre- biased NPN that pulls a GPIO low.