for a controller , take a look at
http://www.dynomotion.com kmotion controller with added snapamp 1000 driver etc
ok not cheap ,uses USB and is configurable & programable for just about everything you need , or would want to do
oh and support for Mach is included
you can pick up shaft encoders on Ebay ( renco rm21 ) for around $85 each , now all you need are some motors and probably a power supply
nice going it'll be a beast once you get it up and running
i'm working on a similar sized co2 laser , retrofiting the electronics
Dave