If you want to keep the old drives then it is likely they are analogue input (+-10v) so you would need to use something like
DSPMC
http://www.vitalsystem.com/web/motion/dspmc.php Galil
http://www.galilmc.com/ K-Motion
http://www.dynomotion.com/Kanalog.htmlall of these are external controllers that interface to Mach via a plugin and prices start around $500 upwards I think.
There is also
YAPSC.
http://max-mod-shop.com/index.php?option=com_content&view=article&id=47:yapsc10v&catid=17:-yapsc&Itemid=4This takes step and direction from Mach via the parallel port and converts it to analogue.
I have no experience with any of the above so cant comment.
Depending on your servos voltage and current you may be able to use some of the drives that can accept Step/Dir signals directly, some of them are
Geckos
http://www.geckodrive.com/Tek10
http://cncteknix.com/portal/index.phpRutex
http://www.rutex.com/us/index.phpViper
http://www.viperservo.com/Gearbox should not be a problem, depends really how it works at the moment.
Spindle drive also will depend how it works, ie is it a VFD or is it a DC Servo or AC Servo etc.
Hood