as ive taken some of the information from other members, and GSF can change there suppliers now and then, if you do get one id check it does have a brass impeller while at the
shop.
the reason people get the water pump done when the cambelt is changed is that the cambelt is removed to change a water pump. Therefore what can be an expensive job to change the water pump on its own due to the labour involved, becomes relatively cheap it done while the belt is off. Thats not to say all water pumps fail, its just sensible to put a new one on 'while your there'.
If you have already done a cambelt change, you may well want to re-consider whether to change the water pump. if it has nto failed. It may well be that it doesnt fail!