All depends on what you want to do really. If you wish to keep the existing amplifiers then you will need to use one of the external controllers that work with Mach and can control analogue amps.
The one I am using on the Chiron VMC I am currently doing is the CSMIO/IP-A from
http://www.cs-lab.eu/en/index.phpThere is also the DSPMC from
http://www.vitalsystem.com/web/motion/dspmc.php and the Kflop with Kanalog from
http://dynomotion.com/KFLOPIf the glass scales are 5v ttl output then you could use them but you may have issues tuning things if you have backlash. The Tach will be used by your existing amplifiers.
If you wish to get new drives capable of Step/Dir input then you could look at the DG3S from
www.cncdrives.com If you go that route then you would not need the tach and could use the glass scales as the encoder input like above but again tuning will not be easy. The other route would be to put encoders on the motors instead of the scales. You could use the parallel port with these drives but personally I would still get an external motion controller. CS-Lab do two step/dir capable ones, the Kflop can be used for step/dir and Vital systems have one as well, think its called HiCon. Then there is also the SmoothStepper (Ethernet version is best option)
I would imagine your existing peripherals such as limits and external switches will be 24v so if you go with the smoothstepper and possibly the kflop you will have to convert to 5v, the others can use 24v I/O.
Hood