I don't know the 560 sl water pump, but I do know that a minor amount of play can be felt in the factory water pumps that go on the 103/104, 112/113,
and 111 engines. And when I say minor, I mean very very little. You'd expect it to be perfectly smooth, but I have experienced a lot that have a very small amount of play right out of the box.
If you see ANY wobble in the pulley then NO it is not o.k., another quick way to check it is to put a stethoscope on the pump while the vehicle is running and see it if sounds rough, clangy, or just not right.