which water pump to buy?

[Qmulus]

There are three general options. Genuine Porsche, Pierburg (OE is also Pierburg, might be the same part as the Porsche supplied pump) and other aftermarket pumps with metal impellers. There are probably other options as well, but those are the only ones I would consider. I would not get a generic pump from the local auto parts store.

The advantage to the metal impeller pump is that if it fails it will not send impeller blades into the coolant passages, which can lead to big problems, like the OE plastic impeller pump. The disadvantage is that if that pump does fail and the bearing fails the impeller can hit the engine case and damage it. I have heard of plastic impeller failures causing big problems, but haven't seen actual metal impeller failure damage, perhaps because not as many are used.

Personally, I got a Pierburg when my original pump failed (bearing started to wobble and make noise, but impeller was OK). From comparing the original and Pierburg pumps, they looked identical down to the markings. In general, I always go with OE parts.