Some people only go BBK on the front to save money. The idea is that the E46 has excess rear braking power, but the ABS deals with it. So yes, you would probably be okay with only going BBK on the front.
If really want BBK and you want to save some money, you can look for a used BBK as they're typically around 50% less than new.
I would still recommend staying closer to stock. I track and autocross pretty hard, and my stock 325 brakes haven't failed me. All I've done is upgrade the pads to axxis ultimates, though I may go more agressive on my next set of pads. Stock blank rotors have served me well. Your 330 ZHP brakes are much bigger than mine.
What would be a nice upgrade is some rotors, Hawk HP+ pads, and even brake cooling ducts.
Also, don't forget that driver/track education is extremely helpful.
BWM Trillium puts on great events that you can write off on your taxes.