I've not had any issues with mine and can't recall a single thread about problems with the AWD system, that's not to say things can't go wrong just that its very unlikely.
It's not a haldex system so I doubt parts will be interchangeable, you might be able to get parts off the RS Audi's which also use a permanent full time system but parts may not be interchangeable.
If it goes wrong you will likely need to get the bits from Europe I expect, even if RS Audi's use the same system there not exactly cheap or numerous so i doubt it will be a simple case of ringing the local scrappy
There are a few threads about 4WD problems - mine among them.
For some the prop bearings have failed - and as it's a welded prop in three parts, with two bearings, that's not been designed to be serviceable, it's a bit of problem finding someone who can.
Others have had problems with the transfer box/power take off. Seems the standard thinking is fit a seond hand replacement - but how do you know the replacement is going to last any amount of time? It's a bog standard ZF 2-stage power take off unit, so I've asked ZF for a parts list and rebuild info. It's not a complicated unit, the failure on mine was the taper roller on the output shaft - a 16 quid bearing - but I need info on the preload needed on reassembly. Just took a day and a half to get the engine etc out to get to it!
3.2 V6 Engine timing chain stretch - I'm taking advantage of the engine out to sort mine. Obviously stretched when you take a look - chains do anyway, it just puts this engine out of oiperating tolerances (as a machinery design engineer, chain stretch is something I have to take into account all the time).
Subframe rust - seems some have had rust holes in the subframe - it appears to be either crap paint, or crap application. Again, due to having the engine out to get to the power take off, I'm having my subframe blasted, after sorting out all the spinnig nut-serts for the undertray etc, and preotecting all the threads, then I'm going to paint it with epoxy paint. I'm not a great fan of powder-coating, which I know others have used, due to seeing too many instances of where the coating has got damaged with a stone chip or similar, allowing water in under the coating. So a quick glance shows lovely shiny coating, but underneath, the metal is rusing merrily until the coating falls off. I'm hoping the paint, applied to a blasted surface, as per instructions, will not do the same.
And I had to sort out the water leaks through the door seals and a leaking body joint.
Other than that, I thinks it's ace
Could job I've owned Italian cars, Lancia and Alfa, since I was 19, so I expect these things