Diana,
The 77 Set still fully serviceable, the other radios were only described to me as casings but not functional (I believe this to be the F1/F3 you mentioned). Unfortunately not a SIG so not entirely sure. The whole rear is wired together as if it was full serviceable.
You are spot on with the seat belts. Every part is as original as humanly possible to keep with the S3. Only non series parts are the British retractable seat belts in the front and the side mirrors. But, I do have the original S3 mirrors in my warehouse of spares that go with the sale.
As for the presentation I am sure Philip would appreciate your words, and yes I have tried to keep it that way, always garaged and cared for.