Before I joined this forum, I had the dealer do the job on my W124. This was over two years ago, and it cost me $850!!
You can save a lot by buying it thru FastLane and doing it yourself, but I will agree with the others that it is a time-consuming job. The serpentine belt is not so much the issue as having to remove the belt tensioner to gain access to the bolts neded to be removed...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car