it's the highest spec, and rarest version of the A7 family...it's a special car, and as such, needs some special attention. Parts are expensive, and not everyone knows how to work on them. They are also absolute beasts to drive (in a good way), with incredible acceleration, and handling that basically flies in the face of it's size and weight. They are truly awesome cars.
I would have the a pre purchase inspection done, as in, pay for it to be done, on the car from a tech who knows these cars. I would also want to know if the TSB was done for the turbo's and I would not buy one without full service history.