@flopes
I am also trying to build a super accurate machine with the Duet.
At the moment i have yet to achieve my target, but it's hopeful.
far as I understand it you'll need 1HCL boards for each motor, and if you want to go closed loop then a magnetic encoder board for the motors.
If you don't go closed loop the only way to get the encoder ticks (at the moment) is to run a M122 B(can address) and parse the result for the encoder tick.
If you do go closed loop, you'll want to tune the values, and spend quite a bit of time tuning at the speeds/accelerations you plan to run.