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I did it on W211 OM642 and there was no fan mounted on the engine. If you have a fan, you have to snake the belt around the blades.
Good idea to replace 2 idler pulleys and even I did not have issue with tensioner, some advise that one too.
The routing usually is in manual.
Is this an OM642? If so, yes, after you remove the fan, there is a tensioner that has what looks like a 'nut' molded into the front cover. You put a (I think) 17mm ratchet on that and pull (really hard) to release the tension. The tensioner has a hefty spring so you really have to put some force into it. Once it moves, the spring tension is easy to overcome.
I did this a couple weeks back at 78,000 miles because I had an oil leak that soaked the belt. The belt was still 'good' (i.e. no cracks) but the pulleys were all coated in dried oil, so I replaced the two idlers, the tensioner and the belt. Once you loop the belt in the correct order, you again will pull the tensioner to allow you to get the belt around it. See photo below for the nut to release the tension.
The picture also shows holding pin at 10-o'clock. Once you move the tensioner, push the pin into the slot and then you can work on the belt with tensioner out of the way.
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