There is a lot of stuff sold on E bay that has zero or questionable value, for the most part used keys from that era are useless. There are some aftermarket programmers that might be able to reprogram but the official MB tool won't.

I'm pretty sure 97 was the last year for a mechanical key with RFID ( I know the system was called DAS X ) To get this system to recognize the remote IR lock, lock the car, wait, press the button twice , open car, insert into ignition and turn to run. ( maybe start the engine, I'm not sure. )

EIS ( Electronic Ignition Switch ) started in 98 - 99 where the " Key " has a rectangular end that inserts into the column lock. The computer reads the key and if correct, releases the engine computer and allows the key to be turned.

There is a emergency mechanical key to open the door if the remote lock battery fails. The battery _IS NOT_ needed to start the car, this is done through the RFID system.