@neilh20 For my sediment machines I use a high torque 920z Nema 17 motor and is rated for 3.36v and 2.1amps. It works very reliably. The one I prototyped is a 76oz Nema 17 which is rated for 4.2v and 1.5amps. I have a pancake motor as well but they are lower torque. Here is a reference to a number of steppers from the 3d printing side of things: here
Generally the biggest issue is to balance the torque of the motor, the fit of the printed pump head, and the feed rate for which you move the motor. In that gcode I provided above, there is two steps to the sampling stage. The pump first moves (G1 E10 F400) tens step at a feed rate of 400. Then is moves faster, (G1 E200 F1400) moving. When reversing the pump it does this same two stage process. I found that it helps reliability and believe it takes full advantage of the torque the motor provides. I could be missing something but this is what I have experienced with this particular pump design.