I have a 2008 3.0TDI Sport Quattro. It's epic. Buy one. That's all you need to know!!
Seriously though, about the only bad thing I've heard about them is that the very early 3.0L models can suffer with premature cam chain wear. Listen for very obvious cam chain rattle on startup when cold. The swirl flaps can also give issues, again with the earlier models which had plastic ones. I believe though that these can be deleted out by a competent tuner and the mechanism removed without causing issues.
I found the starting procedure to be a bit odd at first with the diesels. Even according to the owners manual it can take quite a bit of cranking in cold weather (up to ten seconds!!) before they'll start. It's a bit disconcerting when you're used to a petrol that starts virtually before you've touched the key but once you get used to it then it's OK.
I've never driven an A5 with one but I can't help feeling that the 2.0L would be a bit underpowered - they give away nearly 60bhp to the 3.0L in stock trim and having driven my dad's Skoda Octavia which has the same engine it feels really gutless next to the 3.0L.
The Quattros are always going to have a more complex drivetrain so make sure that the centre Torsen diff has been serviced properly according to schedule.
The only thing that I can fault it on is that when the roads are greasy it tends to understeer massively if you hit a tight corner too fast, even off the gas. That could be something to do with the ridiculously low-profile 20" tyres mine has fitted though... . Fuel consumption isn't bad for such a big motor either - I get about 30mpg average (my old petrol Mk4 Golf 1.8T GTi only got about 26mpg average!!) and up to 42mpg on long motorway trips.
As long as it has good history and drives well then you're unlikely to get many issues.