I have an A3 3.2, had it for almost 5 years now. It's been a fun drive but things to be aware before you dive in:
It's thirsty! If you hoof it around be prepared for low to sub 20MPG. On a motorway, I think 35MPG is the best I have had on a long run. 30MPG is more realistic.
It's quite a twitchy little beast compared to the 325i we had. BMW was much smoother delivering power whereas this feels like every press of the go pedal wants to launch you through the rear window.
I have the Sport edition so ride is pretty firm but it feels totally planted. Your bottle will go before the car loses traction.
If you get a high mileage one, make sure Haldex oil has been changed. That's a few quid otherwise. I just paid £250 at my local indy for the full 80k service (not including Haldex, I had that done earlier). Audi will want around £400 for the full 80k service
I've had a few little bills for pads/discs and a new radiator but other than that it's behaved itself. I will be moving it on next year though and I think given the choice I wouldn't have one again. Fun car to drive, just not relaxing to drive at all, I would choose the 6pot 3 series over it if you can? Neither the Audi or BMW will be cheap to run.
Mondeo ST 220 perhaps cheaper....