The easydrivers will be fine for testing and learning - much better than the Shield. I think their max voltage is 20v.
I am concerned that the shield with 12v might allow too much current through the motors and damage them.
The motors may be fine for the longer term - I certainly wouldn't replace them until I had found them to be insufficient. But a more powerful driver board would probably be desirable - higher voltage and current capability. I suspect the BigEasyDriver would be OK.
If the motors can't power your final project you will probably need ones that take 3 amps or more and then you will need more expensive non-hobby stepper driver boards. You will see some on Ebay that can control 3 or 4 motors. Most of them use the same Arduino connections as the easydriver - but are quite different from the motor shield.
...R