A/C System Renewal - Component Locations?

There is only the single sensor on that system, it shows both high and low pressure through a variable resistance. The expansion valve is inside the HVAC case inside the car. To get to it you have to remove the dash, then the HVAC case and then take the case apart. Personally, I would leave it alone until you test the system and see if the one in there works or not. As you are replacing the compressor because of rust and not because it failed internally it is likely that the TXV is fine. What I would do at this point is to install the new seals and compressor, then pull a vacuum on the system and see if it holds. If it does, then charge it and test the system. That will tell you if the TXV is okay and if the rest of the system works. Then if the TXV were to test bad you could get a replacement evaporator core and heater core and do a rebuild on the HVAC box while it is out. This is especially true on a GM of that vintage because of the plastics they used for the parts liking to break instead of coming apart. Better to risk the damage once than multiple times.

Great, thanks.
I will replace the Compressor and also the Drier and use new seals as well.
Then test the system and see if it keeps vacuum.

Could you tell me as well where I can find the Pressure Switch?

The transducer is in the line next to the port on the AC line.