I won't argue with anyone here, since I never did the job on my W124, but there is a DIY for ball-joint replacement
PeachPartsWiki: How I Replaced the Ball Joints and Tie Rods . I have read that some 124s do have welded in ball joints, but don't know which ones.
That being said, given that complete control arms with new joints and bushings can be had for about $165 a side, I think that is the way to go by far. Less time and aggravation for more improvement.