There are several suspension lowering kits for the S124.
Most of them compromise spring changes, and the new springs let you more or less decide how much lower you want to go (1", 1.5", 2", 2.5"...) It is a simple swap.
Although, be certain that the ride quality will suffer and that the suspension will bottom out faster if the car hits a large pothole.
From what I know, there is no way to play around with the Self Leveling Suspension System (besides over-riding the weight sensor to make it start levelling the car at higher payloads). Anyway, that system is very sensitive and only serves the purpose of levelling the car when loaded (it is not an actice/semi-active suspension system), so I would advise you to leave it alone.