This link should get you going. There is also a thread elsewhere on this message board regarding this project that has notes and FAQ's and tech feedback.
BTW, the mouse idea should be reconsidered. Reason: speed of mouse vs speed potential of DC motor.
The fets in the project above won't switch 250vdc if I recall. Might want to drop down to something more common to lower voltage DC. Study fet specs and consider power supply carefully if you improvise and BE CAREFUL.
Do NOT use a starter motor - way too much current draw and torque. These can get violent when you hit them with 12vdc.
Keep in mind that some motors are designed (internal brush timing) for unidirection motiod. Thus speed is NOT the same for same voltage at different polarity - this poses real control problems.
Don't skimp on preciseness. When you start moving iron so it can cut metal, stuff can break and people can get hurt in the process.
I'd look for surplus servos with fitted encoders before I tried to craft up something myself. It is way to easy to overwhelmed trying to engineer everything (drives, belts, ball screws, power supplies, software, limit switches, etc)....