I'd recommend getting the W124 series CD manual set published by MBUSA. It covers the M119 as installed in the 400E/E420 which is essentially the same engine as in the S420. There's also a good amount of information in this forum on what should be replaced and what can wait for later in terms of guides and tensioners.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD