If you aren't doing it already keep a record of the oil you put in, recording the date, the odometer, and the amount of oil added. That would give you some evidence to present to the dealer and MB. As you haven't had any satisfaction from the dealer go directly to Mercedes-Benz (Ph: 1300 730 200). There shouldn't be any reason to be scared on driving the car, all cars use oil, it's the amount of oil used that it the question that needs to be answered in your case. In the meantime check the oil level weekly.

I have yet to drive the E400 but, in general, mid-sized Mercedes drop-tops have lots of space and comfort but are not remotely sporty. If that’s what you crave, it should do the job.