Hi Hubss
Welcome to the Forum
Not sure which Brembo calliper but you could take a look at www.realoem.com I know the BMW Msport 135M used 340mm discs up front and the Msport Callipers are 4 pot
Remember if you upgrade the calliper you will need more clearance with the wheel 18" would probably be smallest and the offset would need to clear the calliper'
I would look at realoem get the part numbers for the Msport stuff then use Google to see if you can get them at a good price. A friend of one of our sons has a 120d 2006 he managed to find a full set Front and Rear discs, callipers (in Blue) and pads for a sensible £800. The big plus was being Msport parts they all fitted without pain, he was already on 19" wheels.
Dave
Forgot to say when you look at realoem for the part numbers if you double click the partnumber it will call up a list of alternates and the models they are used on