I just want to share my experience that happened to me on my 1993 400E with over 193,000 miles:
A few weeks ago, the ignition key tumbler got stuck and would not turn. Thinking it was a worn key or tumbler, I ordered a new one but the new key didn't work and with the help of my friend, we both could not remove the ignition key tumbler even with the special tool (eBay and makeshift tools).
After spending a lot of money on towing, my friend ended up using a Dremel tool to cut out the metal cap and eventually got the tumbler out. He found that the clip, that holds the black cap, was broken which is why it wouldn't come out with the special tool. He also found that the steering lock assembly was bad as he couldn't unlock the steering wheel even with the tumbler removed. I had to buy diamond point rotary bits so that he can drill into the locking pin to remove the faulty steering lock assembly. So we ended up installing a new steering lock assembly, ignition switch, tumbler, and buzzer switch as well as having a new key.
The 400E was down for about a week and I roughly spent a little over $700 in parts, tools, towing the car, and buying resource videos from Mercedes Source. YouTube videos were not much help at all. I had no warning or issues with the key tumbler such as sticking and it just simply happened out of the blue.
My advice is that if you have never replaced the steering lock assembly on your W124, then I recommend to just do it so you won't get stuck.