Sounds a good plan. If you're needing genuine parts, you could try Audi tradition. They sell NOS Audi parts for older models. They're not cheap though.
I don't think it can as you won't have the mounting for central seatbelt as I'm sure it won't be integrated into the seat and is the type that comes from the roof if that makes sense
Any similar 1.9 will fit. But if it's a different engine code then you will probably need the matching ECU. And having it all coded to your injection pump and instrument cluster