At Tires Too we always recommend replacing the water pump and related components when replacing a timing belt. A 2001 Merc SLk 320 has a timing chain so there is no maintenance interval on those components. If you are concerned about reliability, or have seen any issues, such as coolant leakage, a replacement would be a good idea. As for the spark plugs and and serpentine belt, the spark plugs are due for replacement at 100k and the serpentine belt should be replaced if there are any signs of wear. The poly-v type belt may not have visible cracks but simply wear in the ribs. A belt replacement should be considered as well if there has not been replacement of the belt previously, or for a long interval. Let us know if we can help further!