They didnt release the TQS until after the 1999 facelift, so an S plate one doesnt sound right
the TQS suspension has some improvements over the normal 2.8 quattro, using S4 parts (solid TRE's, additional subframe bracing, different rear wishbones, and others)
Our car had concert, but that went in the bin as its *****.
Our TQS seems to do about 32mpg on a motorway run, and is also more tuneable, with a simple remap easily exceeding the power and torque of the 2.8 motor.
As for your ebay bargain, all i can say is well done. End of the day its a car, on ebay, with no MOT. It could have any manner of things wrong with it, and the price drops as a result.