You will need a vcds or obd11pro , mabey carista to code in battery
But if you will replace like for like ( voltage amps ) you dont need to code anything
You wont be able to operate any aftermarket units as audi a6 c6 sound system is basted on MOST ( fiberoptics ) . Have you got red display or colour above the vents ?
I presume you can start the car ?
When you press the brake pedal there should be a sound of the lach disengaging that blocks the gear leaver. It might be that what is broken