I would agree an A4 S-line of that age will have suspension too harsh for me. I bought an 09 3.0 Tdi Avant and after driving a number of S-line and SE’s, some on 17″, some on 18″ and one on 19″ wheels I eventually found an SE on standard suspension and 17 ” wheels. It was comfortable and, with the badges removed suitably anonymous. I would agree with the opinion above having driven both at the time that an s-line driven back to back on UK roads at anywhere near legal speeds was less enjoyable than the car I had.
But others might think the looks better than the drive experience, or enjoy having the subtly improved turn in the stiffer suspension gives and can live with the ride, I fully understand that.
As far as reliablity is concerned, in the 4 years I owned the A4 I had £550 of repairs completed ( noisy HP fuel pump pullley) of which Audi picked up the bill for labour as goodwill as it was out of warranty so I paid about £250 IIRC.
The Skoda Superb that replaced the Audi I also had for four years and it had a door locking switch on the drivers door replaced. Thats all.