well, its not a certain income level, but you have to look like you could buy a mercedes, or like your parents or trust fund would buy you one. (dress like a yuppie, you shouldnt have any problems)
they dont run your credit just to offer you a test drive...
edit:
sports cars/exotics (911 turbo, etc.) are different, because often there is not a "demo model" per se - and the customer is only allowed to drive it after all the paperwork is filled out, waiting to be signed and delivered.
also, dont forget, there is now the (disgusting) c-class coupe, so mercedes are within the reach of a lot more people than they used to be...