Don't be cheap on such a crucial element, these cars cant afford the water pump to fail. The M54 engine can run very hot quickly (in comparison to other vehicles) and warp the head before you even notice the car is hot. I suggest these brands ranging from cheapest to expensive. FCP Euro hand picks only quality parts if you are confused on what is good quality brand or not. Make sure you are buying a new thermostat at the very least. Tackle all the hoses, plastic pipes, and expansion tank as well since those all eventually crack. FCP euro has everything.
(uses plastic impellor but its the same company that built the one from factory)
(metal impellor)
(the top dog)
BMW High Performance Water Pump - EMP Stewart STE30330
BMW High Performance Water Pump - EMP Stewart STE30330
www.fcpeuro.com