To be honest, if you're using a brushed motor, you can do without the arduino. If you add a linear actuator-style diode and limit switch circuit after the ESC but before the motor, it should just stop the arm from going further than it needs to. You can still use an arduino if you want, it depends where your skills lie as to which one you want to pick. If you want an example of the arduino method though (which I used with a brushless motor) you can check out my build thread (Aegis). I have a video talking about how it works too.