The 1.8 step are plenty accurate enough. Plan B doesn’t actually even use full microstepping.
Bipolar stepper motors (4 wires). If you have unipolar you can still use them as bipolar.
Sidenote about the motors. You need 4 single shaft motors (shaft coming from one side) and 2 double shaft motors (shaft coming from both sides). These 2 double shaft motors are necessary for the big gantry axis and the spreader.