Hello,
I can make a Python based software for you, which can read in the STL file and send the stepper motor controlling parameters for Arduino.
If you like this solution I suggest you to use eg. Raspberry Pi instead of Arduino. Rpi would be a compact solution (PC+controlling unit in one place).
No matter how you decide, I can make the program for both.
If you are interested in my offer don't hesitate to contact me!
Best regards,
Istvan Terjeki