Mine has 14k on it, and the brake pads look just about new. But I live in the flatlands.
On previous BMW's I always replaced the rotors when replacing the pads - something about BMW rotors having less tolerance to resurfacing. Not sure if the 135 is the same, but I would be surprised if it wasn't. Also, I've looked many times at buying from an online shop, but when you figure the shipping (with rotors), the dealer price was close enough. Many dealers will give 10-15% off to BMWCCA members, and sometimes if you just ask for a discount. Don't forget to buy a new sensor wire -you will need to replace yours, as it wears through to trigger your dash light (again, I'm assuming the 135 is like other BMW's -please correct if anyone knows different).
One more observation - if you buy different type pads/rotors, be prepared for the brakes to feel very different than they do now. Harder pads/rotors will require more effort, of course.