Though checks for other components,pipes etc should be made Hard starts on the 300sel are invariably tracked down to two relays not functioning correctly. fuel pump relay and the overvoltage relay. these are in a row with the Klima relay.
remove relays and clean the pins ,or resolder the joints .
Its Best to replace both for long time reliability as they are at the end of their life cycle in these models . The OVP malfunction does not let the cold start voltage through hence the long cranks.
regards
30se