If your AC isn't working you may want to do like I do and that is pull the condenser so its a straight shot from the front. Makes changing the WP a snap. I left the mess off (condenser and fans) and I swear that '87 300D never ran so cool (well, the new radiator helped I'm sure).
I always replace the pulley if its old because the bearings are usually about to go out.
You need a large hex wrench or socket for the idler bolt then some loctite for the bolt when it goes back on real tight, what is it 45 nM?

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!