The original Apple charger has an output limited to 2.1 A at 5.1 V.
I occasionally charge my iPad 2 Wifi with the old charger of my passed Palm Pre. It provides an amperage of 1 A at standard 5 V USB voltage.
I don't know if you can go even lower than 1 A, but the standard USB amperage of .5 A is not enough to charge it. It shouldn't be a problem to get a car charger that is capable of handling 1 A, more is better.
Make sure you buy one, that is capable of charging an iPhone as Apple devices tend to observe the USB standards and try to communicate with the charger. Some dumb China elcheapo chargers may not have USB logic inside and your iDevices could refuse to charge.