Z axis must be fast. ideally I want 50mS for 20mm travel.
12.5 mS to full velocity
maybe use a big NEMA14 stepper with rotor inertia  18g/cm2
Total lift weight  – 16g (carriage) +20g( stepper mtging plate) + 120g (NEMA11 hollow stepper) + juki adaptor and nozzle (36g) + air fittings (20g)  .

call it 220g
Driving sprocket size : 24 teeth of GT2

I plug the numbers into my spreadsheet-
Stepper pull out torque , lifting- 0.093Nm
Rotational acceleration = 5235 rad/sec2
Nozzle change – requires a >700g push down… = ~ 7N on the belt or 0.052Nm on the motor shaft.

Choose NEMA14 (35mm square) with 2x lifting = 2 * 0.093 = 0.186Nm…
http://www.omc-stepperonline.com/download/pdf/14HS13-0804S.pdf

Not included in calcs- rotational inertia of pulleys (not much) , and belt weight (not much) , frictional loads of the MGN9 linear guideway (not much).