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
Welcome to the forum you'll find the members on here are a friendly and helpful bunch 🙂
Good on you for wanting to restore an Audi modern classic. I too am in a similar boat.
What work needs doing?
Not heard of the c8 suffering from this. But can be a number of things from looking at previous A6 models such as blocked water drains allowing water in the cabin, leaking pollen filter housings, leaking sunroofs, leaking door or window seals.