If you're doing it yourself, please forget the other kits and get the Audi OE kit. As to water pumps, Hepu do metal impellers, but they're very hard to source. I got one from GSF last month, but the impeller needed a little modding to ensure it gave a good flow - the casting left a lot to be desired.
Cheers,
Mike
PS Insidentally, most of the impeller damage happens with either very hot coolant or when the coolant freezes and the pump tries to shift ice - the impeller the just breaks up. To avoud this, use a 50:50 mix of Audi OE coolant, called G12+