Get a known brand pump, a cheap Chinese knock off might not last too long and you will be doing it again in no time.
Keep the pulley if you trade it in on a rebuilt pulley, a rebuilt pulley maybe a good buy depending on price, not just any r3builders part though, get a warranty with it. Cardone is a reliable rebuilder, there is not much to replace in a water pump, just the seal and bearing cartridge unless it is really wrecked. Get one specified for an SLK320 NOT a Crossfire as they seem to be marked up too much.