1998 to 2006. You will want to replace this for better belt life if the surface is worn or rough.
This pulley is usually replaced when you replace a worn-out and high-mileage water pump. If your pump is good and you just need to replace the pulley, you can do this very quickly by just loosening and dropping the drive belt.
M112 V6 and M113 V8 Water Pump Metal Pulley
$48.75
SKU:
GEG-M113WPUMPPULLEY 
Product Details
A worn pulley like you see below with shorten the life of your drive belt.
If you need instructions on replacing this pulley, they are included in Kent's water pump instructional video, which can be purchased separately. See related products below:
Related Products:
On Demand Video
$14.99
M112 V6 M113 V8 Water Pump Replacement Instructions - On Demand Video
For most 1998 to 2006 models. A very detailed and helpful DIY long video
In this long 32-minute instructional video, Kent will demonstrate removing a failed water pump and replacing it will a new one on a W211 E500 with the M113 engine.