Hi kf2qd:
The dspmc controller is one I have used for many years, and it is an excellent unit. Another one is made by CS labs, which I have no personal experience, and a third one is Galil.
All of the external controllers have more I/O than you will probably ever be able to use, so that is not a problem.
The dspmc unit is made in Phoenix, Arizona, and they are very responsive to any questions. They have a section in this forum, as well as the other systems mentioned. None of these controllers are cheap.
The only downside to using an older analogue driver system is availability of drives if you need a repair or replacement. Since a big portion of the cost of a system is the servo's and amps, this should be considered. The servo's were brush type DC motors. Retrofitting an older system can be costly in many unseen ways, and there is no "cheap way" to build a good system. Going with a newer controller will also involve fitting newer design shaft encoders of the differential type.
Analogue systems are no longer being manufactured, as the CNC machines have evolved into brushless AC servo's at the high end, with built-in encoders, and PID auto servo tuning software. They are faster, smoother, more accurate, and simpler than the older vintage machines.
It would be worth considering a newer retro-fit to a well built, solid machine foundation. The older machines were usually very well built structurally, and make an excellent starting point for a state-of-art machine. You want to end up with a machine that is stable, trouble free, and easily repaired of need be.
You can do a lot of reading on the respective sections in this forum, regarding the analog systems, and see what the various builders went through to get their machines working. Many have entire machine build histories covering months. Very interesting.
John