I don't know what the acceptable current is, because it must be specified by the manufacturer or found experimentally from temperature. As such, I can't really help the OP.
However, my point is that the resistance is irrelevant when you use a current-control chopper drive because the feedback loop therein will cause the correct current to flow regardless of the stepper's resistance. If you're depending on the resistance to set the current, then the driver circuit is very bad because it must run at an artificially low voltage and therefore will achieve a very low step rate and torque because it cannot achieve a reasonable dI/dt with the resistor soaking up the whole supply voltage.
However! For the purpose of buying a driver, 1A will be plenty. They're pretty small motors by the look of it. You may need to run them at less than 1A to prevent overheat, but I don't think you'll need a driver capable of more than 1A, maybe 2A.