Best bet is to replace the entire ABS unit with a rebuit unit. Get the Bosch (pretty sure it's Bosch) part number off the ABS unit in your car. I say get the Bosch number because I can promise you that same ABS unit is on many different cars, not all BMWs.
Then go to some auto parts stores and see if they have or can get a rebuilt unit. It's the cheapest way because there's a good amount of labor in rebuilding the unit and your mechanic will charger for it.
Only thing you'll need from the old one is the ECU, that black box the with the connector.
Once you have a rebuilt unit it's just a matter of bolting the thing in and refilling the system with brake fluid.