Start with the easy stuff first. Have you checked the distributor cap and rotor? I had a hard starting problem caused by a cracked cap. When was the last time the plugs were replaced? How do the spark plug wires and connectors look?
Check the ignition system first, then fuel delivery/injection, then compression.
Any other odd things happening? Loss of power during acceleration? any dash lights on? Do you smell gas?
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino