I buy all my hydraulic brakes second hand and have done for many years - something like 20 pairs. I never had a problem with any of them. You just need to be sensible when you look at the listings. If they describe them as like new, that's what they'll be like from EBay because you're covered by PayPal if they wrongly describe. Many are taken off new bikes because the owner preferred a different type. Obviously avoid "needs new pads" and "just need bleeding".
Regarding brake cut-offs. There's a regulation that says you need them if your motor doesn't cut within a set time or distance from when you stop pedalling. The cheaper controllers often run on beyond that, so technically you'd need them but no one is checking that, however, some run on for a long time, which renders your brakes completely ineffective. For those, brake cut offs are essential. Also, if you smash your throttle, it goes to half-throttle signal, and if you try to stop the motor with your brakes, you blow the controller. That's why some day cut offs are advisable when you have a throttle