Today I realised that at this height right here, my 190E is basically sitting on the bumpstops 
:
The shocks are Bilstein B8 #22-003652. Surprisingly, it doesn't ride that bad... But I want to fix this ASAP.
When the shock is fully extended, I measured that I have about 4.75" from the top of the shock body to the bottom of the bumpstop. I know that the bumpstop is taller than what we can see in my picture, so I probably have some more travel left. However, the bumpstop is always somewhat compressed in bump and it's rate is now combined to the rate of the spring.
I am waiting for a response from Bilstein to tell me exactly the travel of the shock I have. When I look at the different available bumpstops in those model years, I suspect that the bumpstop I have (129-323-05-44) used to be 83 mm long:
http://www.w124performance.com/images/W124_suspension/stop_buffers2.jpg
With the shape it is in right now, I think it should now be around 70 mm long. If this is correct, that would mean that I have 4.75" of droop travel, 2.75" of bump travel and 7.5" of total travel.
For those Mercedes, apparently you can gain about 1" of extra travel by swapping in extended R129 SL shock mount. I plan on doing this. That would put me at 3.75" bump travel and 3.75" droop.
Now I think it is important that all the bump travel isn't always spent compressing the bumpstop, right? So could I get let's say a 1.75" bumpstop that is made of a hard material and then I would get a full 2" of bump travel that is only spent on the shock? I know I can still get the red 75mm AMG bumpstops in the link above, but as you can guess they are super expensive and hard to find. What is a good source for universal bumpstops? How can I determine what length I need? Is there a preferred material to use?
I experienced a night and day difference in my miata when swapping from standard bumpstops to those sold by FatCat Motorsports, along with new shock mounts. The MCU (microcellular urethane) bumpstops have a progressive rate, rather than immediately shocking the suspension with a very hard rate. Progressive is good here, as it maintains grip and load through the suspension, less jarring. More bump travel is also a very good thing.
Many of these old Mercedes suspension systems are designed so that the bump stop acts as part of the suspension main spring during any sort of bump. It's not a bad thing for a street car. It's comfortable and gives a nice progressive feel as stylngle2003 mentioned, and it's less wear on the hard parts of the suspension to do it this way.
You can actually get a set of new OE replacement 190E bump stops from rockauto and trim them with a knife or hacksaw. Koni and Bilstein also sell generic MCU bumpstops, but they tend to be for motorsports applications and are shorter as a result. The Fat Cat bump stops are actually a Koni part that's been trimmed to a good length for Miata dampers.
As for length, I would start within 10mm of your factory bumpstops and go from there. It might seem weird to have your car basically sitting on its stops all the time but it's a lot less of a problem than it might seem at first.
Thanks for your answers 
I got the answer from Bilstein today, apparently the total travel on this shock is 5.75". This means I have 1" in bump and 4.75" in droop. The springs are Eibach with 1/2 coil cut, so it's not that far off the intended setup to use with those shocks either.
Boxedfox, do you think I should go with cut down OEM bumpstops or I should try to find the more rare, firmer and shorter AMG/500E units? Where did you find the info about the old Mercedes suspensions being designed to ride on the stops all the time?
1" is very short, but that's at the shock. I should measure how it translates at the wheel.
I'm guessing that due to a few factors (including the car being held up by the bump stops a bit) you actually have more than just 1" of bump travel. You're probably right in that it isn't much travel though. A lot of those Eibach lowering springs are pretty low to start with. Cutting another 1/2 coil off of it will cut into that figure even more.
If I was in your shoes I would get the replacement OEM bump stops, trim them to stock length and see how they feel. They aren't quite as durable as the Koni or Bilstein-made MCU bumpstops but they will last at least as long as your dampers.
As for how I happen to know about how these cars' suspension systems are, I have a few friends who really like those cars. And after hearing them expound upon all of the detailed engineering that went into that chassis, I admit they've gotten me curious about them too.
I received the bumps stops this week:
They are the Sportline part number, so they should be a little stiffer than stock. Which end should I cut? Some sources say to cut the top, others recommend cutting the bottom...
Hmm. It depends on the size and shape of the stop. In this case I'd probably choose to cut the bottom (right near where your pinky is). I'm slightly afraid that if you trimmed the rounded top part it would tear due to the slits and odd shapes molded into the stop.
