GradyService; I tried to send you a reply on ball joint replacement, but seems it got lost in cyber-land.
I did a 126 BJ replacement a few years back: worse then a tranny overhaul, but I got-r-done.

From my experience, you will absolutely need to take the steering knuckles off the car: getting the old BJ out, and pressing the new ones in are a tough job, even on the workbench. The knuckle bends around the BJ, and makes use of normal pullers and presses virtually impossible, and the old BJ is REALLY tight in there. You can try to do it on the car, the MB shops probably can with the proper tools. But if you fail, then proceed below.

1. To remove the knuckle
a. For safety , support the lower control arm, and raise it a tad to help support the spring. (With the wheel hanging the only thing holding the spring back is the shock absorber shaft). Look it over carefully, make sure the spring is contained/controlled.
b. Pull off the brakes, hub, bearings, ABS line, etc.
c. Unbolt the tie-rod arm (steering knuckle arm) 2 bolts. No need to pop off the tie rod joint.
d. Remove the nut and pop the upper BJ out of the upper control arm. You will need a small rigid 2-prong puller, like a tie-rod puller (don't mess up the grease boot).
E. The knuckle should be free now, except for the lower BJ. Swing the knuckle out, and get access to the lower BJ nut and shaft. Pop it out using a larger puller, or a pickle fork, or whatever. No worry about destroying the boot.

2. Now you can bring the knuckle to the bench and fight the BJ.
As I remember I hammered on it until I turned blue, I couldn't get it to fit on my hydraulic press. Whatever form of violence works; just get it out. (or see below).

3. Last, you have to press in the new joint - good luck.
I could find no press or tool that would fit, the damn knuckle arm gets in the way. The proper tool is 116 589 04 62 00, a screw-on press with one side cut away to clear the arm of the knuckle.

So, I took my knuckles and BJ to a MB shop that had the tool.
They pressed them for about $50 if I remember correctly. only a few minutes, but I was glad to pay.
On hindsight, I think I would have let them press out the old ones also.

4. The rest is just re-assembly. Torque values
upper BJ nut - 40 NM
lower " - 80
fixed brake caliper bolts (2) 115
floating " 90
tie rod arm bolts (2) 80

Good luck,
DG