I used an ancient 12v charger, and it shows anything from 4 amps at the start to the needle just quivering after it's being going a while, never bothered to measure the voltage. Some really rusted parts I left in for a couple of days, twisting them to get all the rust exposed.
When it comes down to it you're going to be taking a lot longer to build the car than you will be de-rusting donor parts so he time taken doesn't really matter
Matt