The seal seals against a sleeve that goes onto the crankshaft. I found that is easier to get another sleeve, put the seal on the new sleeve, slide the sleeve and seal on the crank, push the new seal in place and then remove the sleeve. Possibly you can get the original sleeve off the crank and use that to install the seal similar to above. Make sure you install the lip of the seal to the rear of the engine. That is why you have to put the seal on the sleeve first because the sleeve will only go into the seal from the larger diameter side.
You have to remove the radiator, at least I did on my 300SD.
You can probably pry the old seal out with a screwdriver.
Socket size is 27mm and should be a deep socket. This socket also fits injectors on the diesels and it is also handy to have to turn the engine by hand for timing, adjusting the valves, etc.
P E H