I definitely would not go for used discs or pads - like what Jeffreyewe said, you dont know what its been thru and can be dangerous. Just not worth it.
As for Brembo/ATE I have not tried before but I do know for a fact that that some models are made outside of Europe.
I am most comfortable with the ori ones and have no complaints - very reliable and confident. Only problem is the high cost. Ori equivalent is my next option if they are made by the same supplier for BMW - should be cheaper. Brake discs are safety items; it has to have the correct chemical compositions during casting and are precision machined. A poor chemical composition (not according to the specified manufacturer requirements) can affect its mechanical strength like metal fatigue, high operating temperature, brake fade, etc. Whereas poor machining can give rise to things like run-out which produces brake judder and unbalance (vibration). It may look like a simple part but the brake disc is a high tech product in which the original equipment manufacturer had conducted extensive tests in various conditions. So, choose carefully your replacement brake discs.