The cigarette lighter will supply on AVERAGE 12v. When the car is off it supplies 12v and it goes down as the car battery starts using up it's power. When running it will be 12v-14.4v because the alternator is supplying the car with voltage.
Amps. Well a car battery can supply around 750 cold cranking amps and about 45aH.
But yes if you want to simulate a cigarette lighter, use 12v and whatever amps you need. And yes people use a pc power supply for this.