I started to get some noise from the front of the engine so I loosened the drive belt enough to spin a few of the pulleys and discovered at least one that was noisy. So after doing some research I found that the 2 idler pulleys and the tension pulley along with the belt tend to need replacing sometime after 100K miles. Mine has 146K, so I went to FCP Euro and bought their kit which includes:
2 INA idler pulleys
1 Febi tensioner
1 MB drive belt
This cost about $165. My Indy wanted 1.5 hours to install, but a buddy said he would help me. When we took the belt off we discovered that it had a 2-inch horizontal tear and two of the pulleys were bad. The job took all of 45 mins. The video at FCP Euro was great and really helped out.

It was very satisfying getting this done!

 

I'm in the same situation. My '08 CLK550 has just at 180k miles. My belt had to be replaced at 120k due to an oil leak from the filter, but it's now time to replace again. Waiting on warmer weather - I'm in Michigan.

 

Same here, I'm waiting on warmer weather to do this myself. When I bought the car 2 months ago the independent Mercedes specialist told me the belts have been changed and but not the tensionsers and they were original
and I needed to change the oil filter housing because it was leaking. I was quoted $750.00 and that is with a discount!
So needless to say i will be doing both myself!