I have some lowering springs to go in my E46 and the dilemma is always, while you're in there you might as well put in some bilsteins, new strut tops, those things that spread the weight better (so you don't get cracking or bowing - I forget the term), wheel alignment, etc, etc and it soon becomes an expensive exercise.
Is your car a motorsport? Mine is, and it already has a mild drop compared to what I can see of your profile pic. I'm going to look at an E46 at the weekend that I'm told is on Z3 springs and that thing is quite 'slammed'. Might be something to look into?