I recently had the same problem with 92 600SEL. Check fuel pressure. On the top middle of the engine under the black rubber harness sleeve you'll find the hex cover (45 degr. towards driver side). Take it off and connect fuel pressure gauge. You can get it from SEARS for $40. When engine is not running and ignition ON the pressure should be around 56 psi. With engine running - 48 psi. Turn ignition OFF and make sure pressure holds for at least 30 min.
My problem was the pressure too high and engine was flooded.
In the trunk find the fuel pump fuse. If i remember correctly, upper right fuse. Take it out and crank the engine, while monitoring the fuel pressure to verify injectors. The pressure should gradually drop as injectors are being opened.
My engine actually started when the pressure was at zero. It ran for second or so and stopped. Next start I had my son to put the fuse back in as soon as engine started and engine kept running. New pressure regulator available here from the Fastline.
Good luck, Mike