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.
You could always get a diagnostic check to see if any fault codes flag up. If you've just bought it from a dealer then they should fix any issues FOC within the last 30 days