E46 330 are a common up-grade,get up-graded discs and pad's callipers,carriers braided hoses their just a bolt on job . I wouldn't bother with the rear as there not as straight forward!
Another cheapish job is to get the brembo callipers from a 750/850 as there 4 pot!
Someone tried to say there're no better but I really can't understand there point as I'd be thinking if you had bigger disc's callipers pads would= greater pad coverage on the disc there for having a greater stopping power!
You might need to chance your break servo to stop the brake peddle travel but people have said you get used to it after a while!