The o-ring is referred to simply as the "timing cover o-ring". The sealant is the same as used for the timing cover. Taking the radiator out isn't absolutely necessary but it will get the hoses out of your way and give much more room to work.

When you take the fan and fan pulley off, you'll find that the fan arbor and bearing are set in an aluminum casting that mounts (4 bolts) to the front of the timing cover, The Bottom right bolt (as you face it) is where the the longest arm of the "Y" bracket for the belt tensioner is fastened. If you take this aluminum casting off (referred to as the fan bearing bracket) you'll see that that bottom right bolt hole in the lower timing cover that it mounts to is slightly larger than the others. This is where the o-ring goes. I have found that the o-ring they give you is too large to seal well around the bolt and too thin to protrude out of the bolt hole opening to provide any kind of seal against the aluminum fan bearing bracket. It's really kind of pointless set up. It is then suggested that sealant be applied around that bolt hole on the mating surfaces of the aluminum bracket and the timing cover. It is further suggested that sealant also be applied on both sides of the "Y" bracket at that bolt hole.
Assuming that there is no oil leak at that hole now, it is likely that the o-ring and sealant is already in place under the bracket and therefore is no need to take the fan bearing bracket off and disturb or replace the o-ring, but it would still be prudent (and/or sufficient) to apply the sealant on either side of the "Y" bracket when you put it back together.
The proper sealant is the mercedes black timing cover silicone. Don't have a part number on hand but any MB parts counter will know what you need. There's a Permatex product that is suitable as well but I don't recall the stock # of that either.

If yo look on the top right side of the fan bearing bracket you'll see two short parallel ridges pointing toward the fan arbor. Bend the last inch or so of a foot long 3/16" rod so it is shy of an "L" shape (about 55-60°). Lay the "foot" of the bent rod in between the two ridges and slide it forward as you turn the fan arbor. It should find a corresponding hole in the arbor to engage in and lock the arbor in place so you can easily unbolt the fan.

As far as the rest of the tensioner procedure, I can't do any better than the excellent article already written up in the DIY section of the forum.