Brake pad replacement advice needed

Hi all - just need a bit of advice regarding the replacement of the rear brake pads on my 330d e92.



They are quite low but the idrive says they have 11000 miles left... No chance! Bmw website says 169 to replace but I can get pads for 30 odd from euro car parts... Would there be any issues with me just buying the pads and getting them fitted at a local garage? Do bmw need to do anything special once they have been replaced a local place couldn't do? Lastly what's the deal regarding the wear sensors? Do I need to replace these? Do the tears have a sensor or each calliper as they only seem to come in singles



Thanks