Have you got it in a garage yet and if so What were the fault codes from the diagnostic check?
If you break it I'm sure till make more than £300 in parts but that's if you've got the space abd time.
It is a Lovely looking TT abd definitely a future classic.
I personally wouldn't have said 136k is high miles for a 54 plate car. It's only 8500 miles per year. My a6 is on 332k miles. So id class yours as low mileage 😉
Cheers
Steve