How much mA does an iPad need to be charged?

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.

A standard USB charge should be enough to charge it, albeit very slowly. It's important to note on a very low charge like this, often it will provide enough to either a) charge when the screen is of, or b) not charge but allow enough to run it without any change in charge levels. In this mode, the charging indicator will always show not charging, as by definition you cannot see it when the display is turned off, but it can happily be charging while you are not using it.