6622.. in the market, it's all brass made, coated in silver paint or brass colour.My advise i have tried the above all, after 6 months, the bearing sound came on and it made lots of noise like a conrod failure.
only now available in the market is the 'local' germany and the original BMW water pump, the stainless steel impeller sold those days are gone, but you might get it in Singapore.
the Local Germany is about RM152.00++ but with a plastic impeller that will last less than 4 months.
The original BMW is about RM496.00 (yes !!) with so called 'black steel' impeller.This will last you 3 years.
My opinion, it's not about the costs of the water pump failing, but the damage if the water pump fails when you are driving, i.e. cylinder head & gasket, mind you that the M52 alloy head and block is very sensitive to overheat. This will cost you thousands in repair bill.
regards.