Same situation as the front , though at the back on the E90 im not sure if its a solid disk or "vented" ? Im not sure what its called . Make sure you get the correct size disk.
I would personally go for either
Fibi Disks or Meyla Disks ...I lean More towards the Meyle but I have never had any problems ever with Fibi Disks ( they were also always cheaper).
And I always use Textar pads . I liked the BMW Pads a lot but they were always very price.And textar makes the BMW pads anyway.
Goldwagen told me that the sensor was R90 for the front , prob R120 for the back as it has a longer wire.
Disks I got from Goldwagen and Pads I got from Diesel Electric and I think
gizmo on the forum could also get you the Meyla Disks, you can PM him.
As a paste , I just use copper compound . I have been using it for years and found no issues . I think spanjaard makes it , it comes in a tube or a can , buy the tube it lasts long and its not as messy.
Try and do the brakes yourself , take a weekend , read about replacing the pads and disks , buy the correct tools and try it out for yourself , If I can do it anyone can seriously , I used to think that it was taboo to work on the pads etc because its a safety item but Honestly its so easy , I now work on my cars disks and pads because I find it more convenient to do it myself and I can do both sides in less then an hour with a little cleaning here and there.
Its not difficult . You must get one of those screw type piston compressor things as well to push the piston back into the caliper to make it easier to fit the new pads and then fit the caliper .
The back pads and disks generally last longer then the front ... well for me that is .
Good luck!