The 1997 E420 does not have caps/rotors. It uses coil on plug ignition. It should not crank for more than 2-3 seconds before starting. If so, there is a problem. There may be a faulty
engine coolant temp sensor or mass air flow sensor giving you
grief. The fuel pressure should be checked. The regulator was updated on those things. You may have plugged filters.
I would suggest taking it to the dealer to have the adaptation
of the fuel trim and the sensor values examined.
You may have a obvious value that is throwing the whole system off.
That may be smarter than shot-gunning parts at it.