Very hard decision to make there is a lot of factors to take in to account when selling ie mileage,service history,equipment levels,Tyre life,Cam belt change as you say,
I would say that given the spend you have made and dependant on mileage sadly you will struggle to recoop the input you have made, i have spent 4-5k just on the car alone with out the cost of buying the car, its a very personal thing is the 8, i can honestly say that i have never seen two that are identical, every one has there taste and adds there little improvements entertainment packs solar ect..
realistic in this market regardless of mileage unless you are a trader selling on finance, your looking at between 4-6k max there are some on PH that are 7k+ but they wont sell for that price when there is plenty for sub5k with full Audi service history ect,
just my 2 pence worth
