Well, this is an interesting one.
The Z3 has mechanical power steering. Had a quick look through the wiring diagrams and could not see any electrical connections. Happy to be proven wrong on this one. Your power steering problems have nothing to do with the battery change.
The windows are controlled by the General Module 4 (ZKE4). It may have trouble codes stored, plus you can check the operation of the windows through this module.
To read the trouble codes and talk to the ZKE4, you need to connect to the 20 pin port under the bonnet/hood and use a scanner with BMW software.
If the people you have taken your car to can't communicate with the car, then there may be other problems. Firstly, get them to see if they can communicate with the engine computer (DME). The DME talks OBD2 and any scanner can talk to it. They can use either the 20 pin port under the bonnet/hood or the OBD2 port under the steering wheel. For the OBD2 port to work, the lid has to be in place on the 20 pin port. If they can talk to the DME, then it comes down to the scanner that they are using to try and talk to the ZKE4.
