I just finished rebuilding my M103. So while I have things fresh in my mind...
I did not replace my valve cover gasket. There did not seem to be any reason to do so.
I also had no need to pull the chain guide pin. It was not in the way of anything.
I had a CLEAN surface on the head and the mating block surface.
Use anti-seize on all bolts.
You don't need a special tool for the front seal.
Use MB sealant A 002 989 73 20 10 for the lower and upper timing chain cover. The dealer has it and it is about $20 and weel worth it.
Do NOT use and sealant on the rubber gasket. Only where it contacts the block. The service manual covers this.
Use no sealant on the valve cover. Make sure the surfaces are clean.
Replace the chain guide and rails and unless you had the timing chain replaced in the six months or so, replace it. No questions asked. It's less than $40 and it will save you much more.
The chain tensioner is easy. The book also outlines that pretty well. There are also recent threads from my rebuild that discuss the process.
Throttle cable is not hard. (Oops, I mean bowden)
'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)