The electric water pump on the N52 fails because of the control electronics over heating, not the impeller. The impeller is ABS plastic (very tough) and is about the size of a half-dollar.
Read up in the maintenance section on the water pump. Find Eninty's post on it. He posted a lot of good info. The pump when it starts to fail will throw codes in the ECU. It's best you get a BMW scan tool to read codes rather than replace the pump and the entire cooling system based on a whim.