If car is not a weekend / owner meets toy I'd suggest to go for 18' genuine Audi alloys. I personally like this ones:
And I'd suggest to buy 245/50 R18 tyres. They are taller then stock 245/45 R18. Will fully occupy wheel arch.
20' tyres are expensive if you go for right stuff like Michelin. And it'll be recurring annual expense as hi performance tyres will wear out quickly. Plus, you'll get tired to change suspension as 20' are much rougher ride.