Ok, CNC retrofits is one of my specialty. I would recommend using a
Gecko Drive 540. They are one of the most advanced stepper motor drives available for the hobbyists. The 540 has 4 motor drives capable of 3.5A each in one fancy enclosure aluminum enclosure.
Here you will find a package deal of the 540, 3 motors, and a power supply. It is the second one down. The motors were designed specially for the 540's motor drivers, and are wound so the inductance match's the drives optimal needs.
2 of the best features of the drive are Mid-band resonance compensation and Auto standby current (70% current). The drives actually morph the output to maintain torque while rpm increases. The second best part is the auto current reduction which lowers holding current to lower motor heating.
As for software
Mach3 is defiantly the best way to go. The free version is able to run 250 lines of code. For $175 you get a license file emailed to you customized with your name. The license moves the line limit up to 10 million lines. You are able to use this license on any number of machines if there non profit. If you are using it for profit, you need a license for each machine.
Your machine would be simple to convert as well. Just create some simple mounts that mount on the existing bearing mounts on the machine, remove the hand wheels and attach some love-joy connectors to the screws and the steppers and finish mounting the steppers.
The 540 is one of the easiest to setup. Just 1 parallel port cable is the connection between the motor controller and the computer. The motors attach with DB9 connectors that are provided with the 540. In the DB9 connector the 4 motor wires attach as well as a current limiting resistor to tell the drive how much current to limit the motor to.
I use some of the 251 Gecko drives which are the same as the drives in the 540, but without the 540 case and breakout board. These drives increased the rapid speed of my machine about 2 times. They are one of the best investments I have made for my machine.
As for motor sizing for your mill. I would not try and use a stepper motor smaller then 200 oz-in. It would not have the power to move the table well. The 387 oz-in motors in that package are more suited to that size of table.
Well I think that covers most of it. If I missed something just ask.
-Adam