Hello Wilde:
Most external CNC controllers with the ability to drive analog amps cost at minimum around $1000.00, or much more. In addition, you will probably have to upgrade the encoders, add a 24v power supply for the controller, a 17" LCD monitor, miscellaneous custom built cables and connectors, and re-build the control panel, or make a new one, plus a good PC. However, when you are finished, you will have a very capable system, with nearly unlimited memory, and a large user base of members available for advice and help.
The improvement in the CNC electronics and control since your mill was built is one of the main reasons to do a retrofit, as most of the older iron was well built, and solid. Also, with a newer controller, adding a 4th. axis is easily done, as they all drive 6 or more axis.
The dspmc by Vital Systems is an excellent external controller, and is the one I used to retrofit an older CNC mill several years ago. There are others, and all of them are capable systems, with many users on the Mach3 forum.
John