First Build 5 x 12 Steel Frame CNC router

[Ger21]

Is this what you have?
http://www.ebay.com/itm/3-Axis-CNC-K...-/231424600113

If so, you're better off re-selling them and taking a loss.

A quality 300oz motor driver kit will typically cost more than that one does. What does that tell you? Do you really think that you're getting 6x the performance for the same price?

As Boyan mentioned, those are high inductance motors. Ideally, you want an inductance of around 2mH. Those motors really need about 100-120V to get the best performance from them. But even then, they won't spin nearly as fast as a much smaller motor.
You generally won't find large motors with low inductance, because they would require very high currents, and very expensive drives.

A few other things to consider. As steppers get larger, their maximum rpm's tend to decrease. I wouldn't be surprised if you can't get much more than 300rpm from those before they run out of torque. Which probably equals around 250ipm with your setup?

Also be aware that you often can't count on the microstep accuracy. Especially with large motors, where the microstep torque may not always be able to overcome the high detent torque those large motors have. When calculating resolution with steppers, 1/2 step is the best that you should hope to achieve.

Personally, I think a machine like that would really need something like 1Kw AC servos. Expect to spend at least $2000-$2500 for three.

As for cutting speeds. I run a Morbidelli at work, and we cut at 1000+ipm every day. Rapids much faster. But this is cutting sheet goods, plywood, MDF, particle board.
Hardwoods are not cut as fast. I typically will cut hardwoods at 300-500ipm with 1/4" and 3/8" bits.

If you're not cutting at these speeds, then you're not taking advantage of your machine and spindle.
One thing that most people don't consider, is that it can be very difficult to hold materials in place at high cutting speeds. My hardwood speeds are limited by my vacuum holding power. If the hardwoods are not held tightly enough, cut quality will suffer, and you'll need to cut slower to get acceptable quality. Slower = shorter tool life.