'03 E320 W211 - Without forewarning of squeals I lost power yesterday after hearing the serpentine belt come off alignment and clunk, clunk, clunk. After a visual inspection I removed the loose belt which looks ok, no noticeable problems with other pulley wheels, just the tensioner pulley wheel - it looks chewed up from the inside and resides loose on the metal bracket where it is attached.

What can you suggest for a proper repair?
New pulley wheel... or entire belt tensioner unit? Should I replace the serpentine belt along with the repair? Anything else to consider with the other pulleys? Also, does it matter if you go with OEM, aftermarket or reconditioned parts?

Thanks,
Matt