Gecko also sells the same drives individually as the G251, but you would then also need one of Peter's excellent Break Out Boards.
I don;t know if the 540 has any relays in it, but you would want to consider that to switch the spindle on and off. Peter's BOB had relays on it and you use those to swithc inexpensive AC relays to turn the spindle on and off.
What is the voltage spec on your motors? 4 wire or 8 wire?
There are different ways to wire up bipolar steppers. Make sure the low end Geckos have enough capacity for what you want to do.
The power can be switching type or unregulated for steppers, but remember that steppers should run many, many times their rated voltage. Read some of the excellent info on the Gecko site for sizing power supplies.
I first ran my 4V steppers at 36V and it was pretty lame and stalled easily. I switched to 72V and it was literally as if I had replaced the steppers with twice as powereful motors. No more stalling.