You can build a complete OnStep system just for the focuser, but there are a number of projects on the internet that just use an Arduino Nano and a stepper driver and maybe a few other components.  Check out the MyFocuserPro 2 project at https://sourceforge.net/projects/arduinoascomfocuserpro2diy/  .  There are several versions presented, including a 'cadillac' versions with lots of bells and whistles and button control, or a simple version that will just communicate with an ASCOM or INDI driver.