You can take the Side cover of the Oil Pan off with the engine in the MB.
AND you may be able to get the Timing cover off "In situ" (Engine in Chassis)
BUT! As you can see R+Ring the Engine from the chassis is the way to go... And still stay sane!
(Me? I'd have the Whole Engine/Tranny system out,might be some parts that need "Freshening Up" in the Tranny system)
Here are gsxr's Travails with a "Regular" OM603 (As opposed to a 3.5L) [Same.Same Except 3.5L is "Bored Out" by MB] :
OM603: Freak failure of timing cover. 1987 300D
AND
OM603: Vacuum pump explosion, bearing intact! Injection Pump Timing device wrecked
his updated URL (Pictures) :
http://www.w124performance.com/images/OM603_timing_cover/
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The Balancing act/Decision to be performed is:
1.R+R and Repair ( Disassemble and Totally clean the 3.5L) then rebuild it.
You'll come to whether this is practical for you as you talk to others who've
Done it.
AND How Much of a Rebuild you're in for.
2.Swap a "Known Good" OM603 3.0L in for the Damaged OM 603 3.5L.
3.Go Crazy and swap a "Known Good" OM 606 Turbodiesel into that Beautiful W126.
You'd use a Mechanical Injection Pump from a OM603 to do away with Almost All
of the OM606 Engine's Computer Crap.