I'm a newbie and know nothing about MB in general and only a tiny bit about my wife's 190sl. However I've been an amateur mechanic for over 50 years and the solution to your quests should not be too difficult.
I am learning, however, about the quirks of these cars. For example I just found that in order to change the fan belt the engine had to be raised about 1/4" because the harmonic balancer is too close to the front cross member.
A valve spring compressor is a generic, very common tool. Here in California some auto parts stores loan them for a deposit equal to the value of the tool. They are probably abundant on ebay also for less than you would expect to rent one for.
If the nut you wish to remove is large, in the large metric sizes there is often an inch wrench that is very close. Again, I know next to nothing about MB but whenever I've removed a differential pinion nut to replace the seal I've first marked its exact position on the pinion so it can be put back exactly as I found it.
Some have a minimum required torque, often over 200 ft.-lbs. I've made an extender for my 1/2" drive torque wrench that both doubles the torque and adapts on 3/4" drive sockets. If a photo of that would be helpful I'd be glad to send one if you give me an email address to use. Mine is firstname.lastname@example.org
I'm not a "purist" and I've found that the alleged need for a "special tool" is often an exaggeration used to discourage people from the do-it-yourself approach.