Having the same vehicle as yourself let me tell you my experience regarding adjusting the steering. One mechanic told me by looking at it that it was adjusted all the way out. Since the car only has 90,000 miles on it I couldn't believe it was adjusted out. My '82 300d has 275,000 miles on it and it still has adjustment left. This one just had to have more left in it.
Using this logic, I took it to the local MB dealership and watched the mechanic try to make the adjustment. He did get to it, but was unable to adjust it out any further. Cost me about a hundred bucks for that experience.
There is absolutely no space to get into the box--as you probably know--without doing some finagling. In this case, he wound up loosening a motor mount and jacking the motor up. He then placed a 2 x 4 block between the frame and the engine to give him a little workspace. Then, he was able to get to it from the top, but it was still extremely difficult and time consuming.
As was suggested above, he used a crow's foot and an allen wrench to make the adjustment. If you attempt this yourself, I wish you luck. He had a lift, a jack, and all the right tools, plus some know-how and it was still tough to do. One tip here is you need to mark the nut with some white paint or something to use as a frame of reference. Make sure you don't adjust it too tightly. If you do, the steering wheel won't straighten on it's own, but you'll have to turn the wheels back yourself.
I have 90,000 miles on my '85 500SL and the steering box was shot. I got a rebuilt box from ************************ for about $325.00 w/a 15% discount and a core charge of $125.00. Labor to replace it was about $400.00 BECAUSE they had to drop the exhaust pipes since they surround the box. In my case there was one bolt that wouldn't release and they wound up drilling it out with LOT of difficulty. Unfortunately, I had to pay for that difficulty.
To sum this up, try to figure out if you have any adjustment left. If it looks like there isn't or if it looks like it might be close, and you're paying to have it done, then I recommend you go ahead and replace the box. In the long run it might save you a few bucks.
Since I haven't come across many Euro '85 500SL's on this thread let me tell you what I've done to mine and, again, it only has 90,000 miles on it. I've replaced the front calipers, pads, tie rod ends, drag link, steering box, fuel pump, fuel accumulator, idle control unit, idle control valve, steering box, climate control unit, overload protection relay, and put a new soft top and rubber on it. Believe I've finally gotten it to a point where I can see daylight. Good think I got the car for a decent price since I've put about $4,000 more bucks into it.
As my son said right before we turned the corner when he took me to look at the car in Houston, "Dad, this car is a head turner!" He was right.