Well sounds like you will have to deal with the electrical portion of the key switch one way or another. Or something in that area. Take a meter or test light.

Go between a ground and the small terminal on the stater. Have someone put the key into the start position. No voltage present is a pretty good indicator of that electrical portion of your key switch is out.

You can run a temporary wire from that small terminal and when you want to start just touch the positive terminal of the battery. The car is so old there are no interlocks to prevent starting on the manual cars.

In the old days we just bridged the small terminal on our starters to the large hot post on the starter with a screwdriver to power up the starter solinoid.

I would carefully inspect the keyswitch area just in case the last owner messed up something. You may want to consider cleaning up the major grounds and battery terminals first. These cars are old and connections can corrode. Wiggling the shift lever for example may have completed a ground for a moment. Again there are just no interlocks on a 240d.