I found it very simple to adjust the valves once the injectors were pulled out. if your 27mm socket is a deep, it will be simple to remove the injectors first. this is not needed, as with finesse you can easily find the space to adjust the valves. I ground the face of my cheapo 14mm wrench, down to about 3/16 " and used that wrench as the lower adjuster, and held the upper adjuster with a normal wrench. put the wrench on the upper part, rotate it to the rear, put the wrench on the lower adjuster, and push it to the front of the motor to increase your gap, and toward the rear of the motor to tighten the gap. then snug up the upper adjuster. measure and repeat. oh, the cam lobes do not have to be pointed straight up, just slightly up from level is enough to measure gap. this way you can adjust more at a time between turning the motor.
I was pulling my injectors anyway due to some leaking and such. with the injectors out VERY VERY SIMPLE to get to everything to adjust valves. plus it is easier to turn the motor over with the injectors out, cause no compression pressure is built up.
be sure your feeler guage set has .10 mm in it. most do not. .04" is pretty close. .1mm for intake, and .30mm exhaust N/A, and .35MM Turbo
this thread has lots of comments and such.
W123. Adjusting Valves the easy way. Advise from a Mercedes master to an apprentice