I did this myself and wouldn't label it impossible, but it was quite time consuming. To get to the lower balljoint you have to remove the steering knuckle/spindle assembly. This means the brake caliper has to come off (I also removed the disc rotor from the spindle for better access) and the upper and lower balljoints have to be separated from the control arms. The steering tie rod brackets unbolted without much drama. I then used a lever style balljoint separater (pickle forks were not sufficient) to free the upper balljoint, followed by the lower. Then I tapped out the lower balljoints using a large ratchet socket and heavy mallet.
Pressing in the new lower balljoints was the frustrating part. The Mercedes shop manual calls for a specialized balljoint press; the closest thing I could rent at AutoZone had a clearance issue. After several unfruitful attempts, I turned to a local machine shop which charged $20 for the service. They destroyed one of the boots in the process so I had to go back to the dealer for a replacement. The reassembly process went much smoother, and the results were a much tighter front end.
I completely rebuilt my 126 SEC front suspension last summer, including the lower ball joints. Earlier response is correct - the whole suspension needs to be taken down to free the lower control arm. Getting the ball joint out is not easy - my brother and I also used a large socket (about 1 1/2" socket) held by large pliers while I whacked the top of the socket with a 10lb sledge. It helped greatly heating the outer ball joint casing on the control arm with a torch. I rented an MB ball joint press from Performance Products for $50 and pressing the new ball joints in was a 1 minute process with this indespensible tool. DON'T try this without this tool - you'll be looking for a machine shop without it.
While you're replacing the ball joints, might as well replace the rubber bushings at the other end of the lower control arms, replace the steering tie rods, center link, idler arm bushings, wheel bearings, shocks, track rod bushings and upper control arms......
Total I spent about $1,200 on parts (accumulating parts off Ebay mostly) and save well over $1,000 in labor doing the job myself. Of course, my brother has a race shop, so I had access to him and his tools.
Nasty, tough, dirty laborious job, but makes a huge difference.
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