I gave up after seveal attmpts to change timing chain on my 400E, so far this is the only job that I failed to accomplish by DIY.
It took me one day just to remove the valve covers, I failed partly because the extremely tight engine bay of 400E.
As I recalled one member has done this together with his mechanic, he indicated it was not practical for one man to do the chain job, but that's for 400E, your W140 may have more rooms than V8 W124.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.