I'm currently looking for the same.
So a must is Haldex 38k, STronic 38k. Brake fluid every 19k/2 years.
The usual servicing, I'd certainly want it done at least every 19k/2 years.
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Nothing really to look for in terms of mechanics in my opinion, biggest could be hesitation in STronic or clutch slip/ flywheel judder on the manuals. Both pretty expensive jobs, well north of £1k.
Also, brakes aren't cheap, nor are tyres, so make sure there is plenty of life left. Set of tyres, £600 fitted. Set of pads and discs, £900.
Above are Audi prices, FYI.
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Finally, spec, spec is everything, there are RS's with boggo spec and some with all the options, but they price similar, don't fall into this trap.
There's loads of specable stuff.
The main are:
auto/manual (personal preference)
buckets
sat nav