I'm not going to try to specify exactly what parts to buy, but I'll give you a general idea of what is needed. I'll give you links to the first hit I get on ebay, you have to determine if the parts will fit your machine. These are not recommendations, but rather just pointing you in the right direction.
First, if you are not going to use ball screws, then NEMA 34 motors will be needed for extra torque, something like this
http://www.ebay.com/itm/US-Ship-3-A...277906?hash=item35e7ec1e92:g:ljoAAOSwHnFVxC0-
The above come with the breakout board and cable for Mach3. I think the 1800 oz/in stepper is big enough to operate the Z axis. But you may have to counter weight it.
Then you need a way to couple the motors to the lead screws. Something like this:
http://www.ebay.com/itm/5mm-x-6-35m...hash=item2a7010bb49:m:m9QujlhH9qTtJLUSb5X9j8Q
You could buy a ball screws and reduce the X and Y motor sizes. A quick Google search indicates that NEMA 23, 425 oz/in steppers might be OK on the X and Y axis if you are using ball screws. Ball screws will give you much more accuracy.
http://www.ebay.com/itm/Zyltech-Ant...918611?hash=item4b0c5c3c93:g:jQMAAOSw2x1XL6Wx
There are full kits available for your machine, here is one vendor:
http://www.cncfusion.com/index.html
Then you would have to make mounting brackets for the motors. Then also you would need a few limit switches for the travel limits.
http://www.ebay.com/itm/5pcs-PACK-M...647537?hash=item1c6c7454f1:g:8UcAAOSwa~BYRQFH
Then some wiring, and other cables, and an E-stop switch something like this:
http://www.ebay.com/itm/22MM-Emerge...109626?hash=item463dd5737a:g:ROcAAOSwjVVVn-8X
Then some type of a box to put all of the electronic s in.
I hope this gets you started.
Sounds like a fun project