Important to find all the broken parts of the impeller. If you don't you can be sure they are obstructing coolant flow.When I rebuilt my M96 I found impeller bits in the oil/coolant cooler that probably rendered it useless and certainly restricted flow. Who knows where else in this huge cooling system the plastic bits might lodge. The cost of the potential damage is eye watering. So routine replacement before failure as others recommend is wise advice.
DOn't forget to buy the correct O rings for the cooler !

The water pump....
 

....also “Times Out”.

You need to replace them every four years no matter how much or how little mileage you put on the car.

The plastic impellers just don’t live longer than five years in the coolant.

And, yes, you want to put an OEM water pump right back in there with plastic impellers again.

You must NEVER install an aftermarket water pump with metal impellers.

The reason why is that the metal blades will carve out the aluminum engine case if your water pump bearing fails. The plastic OEM water pump will not destroy the engine case if the bearing fails.

Just factor in the cost of an OEM water pump R&R every four years into the maintenance cost of owning a 986.

Cheers.