Hi Jon welcome to the forum :)
You'll find the members on here are a friendly and helpful bunch :)
The a4 b8 is a good car and can provide reliable service if it's been well maintained and looked after. but a few recorded problems are as follows:
High oil consumption
Vibration through the steering wheel at low speeds
High pressure fuel pump failure (mainly older examples)
Issues with injectors on some models
With any car you are considering purchasing I would advise you look at having an independent vehicle inspection carried out. Companies such as the AA and RAC offer this service for a fee.
If you ask the seller if you can get an inspection carried out and they refuse or is reluctant then you have to question what the seller or the car has to hide.
Also on any purchase check the service history and ideally this should be full. Any car that 5 years or older or with 70k or more on the clock may be due a cambelt and water pump change. This will cost any where from £400-£600 depending on what parts and garage are used. Audi often has offers to have this done by them.
Also check to see that all electronic items work as some of them can be expensive to fix. In connection to this sunroofs are desirable and check to see they operate properly. The sunroof also can cause a lot of noise and buffeting when open, more so than other similar cars.
You'll also find that Bluetooth was an optional extra and if it's important to you then make sure your potential purchase has it as it is very hard to retro fit.
Spec wise s line is a good spec as you get better seats, more toys, nicer wheels or lowered suspension. Obviously the higher models then s line are better still :)
Here's a YouTube video iv found:
Hope IV helped :)
Cheers
Steve