Firstly, I assume you mean 'milling bit' rather than 'drill bit'? A milling bit can cut sideways (radially), whereas a drill can only cut straight down (axially).
Step and Direction
There are two types of stepper motor drivers.
One type has two digital inputs, 'step' and 'direction'. The controller makes the motor move one step by sending a pulse (high then low) to the step input. The more often it sends pulses, the faster the motor rotates.
The other type has four digital inputs '1', '2', '3' and '4'. To make the motor rotate is slightly more complex:
The board you've chosen uses an L298 stepper motor driver IC. This is one of the 4-input types.
I assume you're using a PC running Mach3 as the controller. I believe (though I may be wrong) that Mach3 only supports the step & direction type of stepper controllers.
The other thing about the L298 is that is doesn't support microstepping. This isn't strictly necessary, but it's a great thing to have. A microstepping motor driver will actually be able to rotate the stepper motor by fractions of a step, often as small as 1/16th of a step. It does this by carefully controlling the current in the motor's windings in a sin / cosine fashion. As you can imagine, this improves the resolution of your system greatly. But it also has other benefits. It actually improves motor acceleration, and prevents strange resonance effects you can get with steppers at certain speeds.
Take a look on eBay some more. Definitely try to get a board with step & direction inputs. And if you can, get one with microstepping, even better.