Metlcutr55:
As I understand your post, the problem with your 20 year old mill is the old DOS computer. If the motors and servo amplifiers are not burned out or malfunctioning there is no pressing need to rip them out, or change them for some other make. They have lasted this long, and are also sized correctly for the machine requirements, i.e., mass and weight of table and cutting loads.
Machmotion makes several levels of machine kit/retrofit systems. Stepper motors and amplifiers are far cheaper than servo motor/amplifier encoder feedback systems. That is reflected in the final cost you see for the different kits.
The dspmc controller used in the Machmotion 1000 controller costs over a thousand dollars, all by itself. It is a highly sophisticated, state of the art electronic system. It does all the motion control mathematics and feedback handling as a totally separate entity from Mach3. The two communicate, but the dspmc does all the "heavy lifting."
You can keep your current Westamp motors/amplifiers/power supply, and utilize a Machmotion control kit with the screen/keyboard/controller and just wire up the existing encoder wires, reference switches, and analog signal wiring to the controller 1000, and you will have a working system. The Westamp limit switches are already wired into the amplifiers, so you do not even need to worry about them.
My Westamp motors and amps are easier to tune, smoother and faster than the original system. My rapid on the X axis is over 220 ipm on a 52" X 11" table, and I personally do not like to try to jam the machine at the highest speeds possible. This is not the fastest speed possible for a mill, but it works fine for all my machine uses. I tend to run my machines conservatively anyhow.
If later you wish to upgrade to newer servo amps and brushless AC motors, you can do it, but it is not a necessity to have a working machine. The newer AC drives and motors are faster, more powerful, and of course off-the-shelf, but that is all a personal choice. If you decide to change motors and amps, you will be into making new motor mounts, couplings, motor power cabling, new seal-tite cabling,new encoder cabling and wiring. All of this is in addition to the basic cost of the parts. Not a trivial task.
If you choose to go with the touch screen option that Machmotion offers, you will have a system that is akin to the best industry has to offer. A touch screen plus an MPG will result in very user-friendly machine, and they look great too!