I don't have experience with this particular box, but I have messed with the 123 box. First of all I expect that it is indeed the box. You can turn the input shaft while your other hand is on the Pitman arm to verify the box as the source of slack.
There is an adjustment on the top that appears to be identical to an American steering box. You will need to hold the allen in place while loosening the locknut. Then turn the allen slowly counterclockwise until you feel the slightest amount of resistance. Hold the allen in this exact position while tightening the locknut.
MB recommends that you do not adjust the box, but I have done it many times, not always successfully. I can adjust an American box and get it right most every time, but he Benz box is elusive. As far as rebuilding it yourself, it is challenging but possible. I got a salvage yard box and did exploratory surgery, then figured out how to shim mine to get it right. There was much trial and error, and you need to ALWAYS err on the side of looseness. If you get it too tight it is a safety issue.