Failing idler/tension pulleys can cause many issues with these cars including overheating, serpentine belt destruction, no A/C, no charging, etc. Replacing the pulleys in the car every 60,000 miles will insure these items do not happen. The pulleys are plastic and have a tendency to wear out and make noise.
Tensioners (Pulley #1 and #2)
There are two types of tensioners; hydraulic and mechanical. Everyone has different opinions on which one is better. The benefits of a hydraulic tensioner is you only have to change the pulley, while the mechanical tensioner requires the whole unit to be replaced. I have never trusted tensioners in any vehicle I own and just change the whole thing every time.
Idler Pulley (Pulley #3)
The idler pulley is there for belt routing and tension only. There is one bolt to remove it. The bolt holding the idler pulley in place also serves as 1 of the 2 bolts to mount the alternator.
All BMW E46s with the M54 share this part.