Paul,
If it's like my 86 300E, the secret is to have two little essential tools: the fan belt service wrench and the fan clutch lock tool. Both of these are available from Performance Products (and possible from FastLane but you'd have to call Phil and ask).
The part numbers and prices from Performance Products are as follows:
Fan Belt Service Wrench - #58-019 ........$7.45
(this is a one-piece stubby 8mm hex wrench with a 3/8" drive)
Fan Clutch Lock Tool - #58-021 .............$16.00
[this is a sturdy piece of bent rod that is designed to jam the fan clutch so the nut can be tightened or loosened. I found a picture from ZDMAK Tools that I've attached below. If I get a chance I'll take a photo of my fan belt service wrench (8mm stubby hex) and post it later]
You can reach Performance Products at 1-800-243-1220 or at
www.performanceproducts on the web. Do a search on those two part numbers.
You can also check ZDMak Tools at
www.zdmak.com and click on "Mercedes Tools/All Mercedes Tools", scroll down to the bottom and click on the 2nd page link. Go to the 9th item and you'll see a picture of the fan clutch lock tool. They sell it for $13.50
I just found ZDMAK while researching this answer for you and they seem to have an awful lot of special tools for our cars at fairly reasonable prices, considering the complexity of some of the tools. Might be worth browsing through the site just to become aware of what they have for future jobs.
Hope this helps,
Gary