Trailer brakes? What does electric over hydraulic mean?

I would buy the used one, and install it, test it at least a few days before any planned trips.

You would want to use #10 wire to the controller input 12 volts, and run a #10 wire to the back of the RV if you can. I would use a 30 amp automatic re-set circuit breaker on the #10 wire. That way if there is a short in one of the brakes, it will reset at some point.

#10 wire will not allow as much voltage drop as the smaller and more common #12 or #14 wires.