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.
Keep us posted on how you get on. If they confirm the car hasn't got b&o I doubt they'll retrofit it as Audi don't do retrofits. Your only option would be to reject the vehicle.
Glad there's no leak. Be careful not to overfill the engine with oil.
Let's hope it's just the service light. If dipstick is normal, but onboard computer saying it's low then most likely the level sensor is faulty.
I think it was standard with the tech pack. My advice is to ask the dealer selling it. If you want a built in sat nav you'll.wat a car that already has it as activating it can only be done by Audi at a cost of £500-600 at least