I am happy to hear that you have been "convertible top malady free."
Your best course with brakes is to replace with OEM parts. I-drive is accurate regarding pad wear and replacement. There is a "closed" electrical circuit on the wear sensors. When pads wear down the plastic end of the wiring also begins to wear. When the bulb wears through and the wiring is also worn through, the circuit is opened and you get the service brakes message.
Brake squeeking is typically due to something happening on the back of the pad. OEM pads have a nice rubber backing that that eliminates the metal to metal squeeking that can occur from aftermarket pads that do not have the rubber backing. If your brake noise is not squeeking but grinding, it may be due to the PO replacing pads with aftermarket metallic pads and the noise would be coming from the pad itself.
Several BMW's ago, I had a 735iL and the first time I changed brakes I went aftermarket. I took the pads off within a month and installed OEM. After 15 years of always having at least one BMW in the family I have never used anything except OEM and I have never had any problems with brake operation or noise. I have found that I typically need to replace the rotor on the second pad change.
Thanks for your response and I hope you get your brake noise resolved. Those types of noises are very annoying.