There are a lot of after market water pumps out there that use reinforced plastic impellers. The GT3 engine block is different. Use a high quality one with a plastic impeller. Pierburg is a good choice - for example PIERBURG 7.31081.02.0. Or Laso 7520 0106. But there are also other good quality water pumps available. And do yourself a favour and don't buy the cheapest.
Regards, Markus