Benzmac reported a good ways back that the rule is "zero preload-zero play". I take that to mean the hub can be turned without pulling a fish scale off of 0 and that there's no play when pushing-pulling from 12 o'clock and 6 o'clock.
If you feel you need a dial indicator to get this, then buy one. I've done it without one and have continued to live for quite sometime. There's no question that faulty wheel bearing work can put one in the grave, but this can happen on ANY car; not just an MB.
What I'm talking about in my opinion applies to the front end. I'm not sure if you really need a dial gauge for the rear.
If you're not experienced in wheel bearing repack and the subseq. installation of the bearings, you might want to find something besides a 126 to cut your teeth on. If your pal is well-versed, go for it, but do so at your own risk.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.