Does the W203 not run a CAN/BUS system? If so there would be a bunch of work involved to allow the ecu to function fully without the transmission communication. It would be best if you had the schematics for the car (FSM) so you would be able to tell exactly where signals from the transmission controller originate and where they lead to.
It might even end up being more worth your while to start with a manual car, whichever it may be and go from there. I'm not well versed on the CAN/BUS setup.
The older cars with CIS have it easy. There is one connector to the transmission with 5 wires, the two smallest are connected to the manual backup switch while the park/neutural switch is wired to a button on the clutch or just separate via a relay. This allows the relay to be open while driving, removing the revlimiter and all that jazz, and closed while starting, allowing cranking of the engine.

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1993 190E 2.6 Sportline