The only difference between higher and lower octane is the gasoline's resistance to pre-ignition (detonation/pinging). Your engines compression ratio is directly related to what gasoline is required for safe operation. Will the car still run on lower octane? Yes. Will there possibly be detonation and will you run the risk of ruining valves, pistons, internal engine components over time? Yes. Plus I think the W124 has a very rudimentary knock detection and retardation system. I wouldn't chance it. Saving ~$5 per fill-up is not worth destroying your engine over.
