Hi FandZ. Marvelous design! Those look to be some really great kids you have there.
Let's talk about that wide gantry. Are you SURE that you can drive it reliably with a screw in the center? It wouldn't take very much misalignment to cause binding. Most gantries that wide use a slaved stepper on each side to get good performance. This requires two drivers, but Mach3 sees both as one axis. Two slaved motors could be much smaller, faster, and more likely be in the G540 range.
Let's talk stepper motors. The Torque rating is what you get with the motor at rest. Torque falls off with increase in RPMs. To do any WORK with it, you need to carry much of that torque up to higher RPMs.
It is important to match the motor to the load. You can't just assume that bigger is better. Bigger motors run somewhat slower than smaller motors. A router, more so than a mill, needs high rapid speeds. You will get best performance by wiring the motors in Bipolar Parallel.
The way to get best rapid speeds is to be able to get torque at high RPMs. This is accomplished by matching the motor's best voltage to the power supply voltage. Higher voltage pulses charge the coils more quickly and maintain torque to faster speeds.
Using the G540 as the controller, (and you should, it's the most bang for the buck) You can operate with a max voltage of 50V. Formula for best voltage of a stepper motor is 32 times the square root of the inductance. With the G540, you will want motors with best Voltage between 50 and 65V.
Let's look at that 495 Oz motor. It has an inductance of 7 mH. Using the formula, it has a best Voltage of 85V. Fifty Volts will seriously under-power it, resulting in slower rapids. A $180 72V PSU would run it faster, but then you would need to go with G203Vs to get the same features available in the G540 and that would be way more expensive. ($560 just for them)
I would recommend the $39 60V Keling 270 OZ KL23H276-28-4B. It's already wired Bipolar Parallel internally, so only 4 wires to connect.
http://kelinginc.net/KL23H276-28-4B.pdf
Three of these would be very efficient for X and Y, and, depending on weight of your router head, might also be OK for the Z axis. If not, then the currently out-of-stock $49 65V Keling 387 ouncer would be your best bet.
The $60 Keling 48V 7.3A PSU would power these very nicely.
CR.