I used hybrid closed loop steppers with encoders on both Z and X axis.
These were ordered from steppersonline via Ebay. They look to be rebranded Leadshine products.
Z axis is 8.25Nm (1200 oz in), and X is 4.25Nm (600 oz in). I am certain that these are overkill, but would rather have too much power than too little.
These require 48V DC, so I will add an extra 20A power supply to power these.
They will be geared via timing belts. This should give better accuracy and increased power.
I had a few issues getting the Acorn controller to work with the stepper drivers, but eventually I got it all sorted out.
When I was getting frustrated, I got great support from the Acorn Forum, as well as from technical support from steppersonline.com.
It turned out that I misread the wiring diagram, I forgot a few wires!