How long have you had it and did you buy it from a dealer? If bought from a dealer in the last 30 days it's their responsibility to repair it free of charge as you can return the car for a full refund In that time.
I'd recommend watching YouTube buyers guides. We've sent issues with the A7 hybrid alternators, I appreciate this is a sq5 but it'll be similar technology:
The old interval was 5 years or 120k miles which ever comes first I believe. However Audi changed this to no time frame. I wouldn't risk it over 5 years with a belt in my opinion.
Itl need a diagnostic check to see if any fault codes flag up. The airbag warning light could just be due to a loose airbag wire under the passenger seat.
Yep I appreciate that, but if both sensors are working and picking up a fault between them then the natural assumption will get the cat. Hence why the car believes it's the cat.
Feel free to change the other sensor and let us know the outcome.
Id recommend getting the car in for a diagnostic check to see if any fault codes flag up.
On my daily (ford Kuga) that has a dab radio I find on frosty days the signal is worse or non existent for some reason
Think the car tax changed only affect new cars I understand. But happily proven wrong.
As Cliff has said service history is key. Proof of cambelt and water pump changes should be seen on cars of that age plus proof of gearbox oil changes etc.
Dpf and egr can fail on diesels and are £1k plus bills.
The amount of mileage and type of mileage will determine whether petrol or diesel is better for you.