I have driven several Audi S8, both the V10 and the later V8 models. They are a superb, well engineered sports saloon, with exceptional effortless performance.
The cabin area is a luxurious place to be, spacious, extremely comfortable, and affords refined luxury performance motoring. They are no doubt a 'big' vehicle, but until you have actually driven one, you don't quite appreciate all their qualities. Running costs will be high, but if you are considering one, then presumably you have worked your sums out!
An near full Audi history suggests that the vehicle has been well maintained (no complete guarantee of course, but a good indication), and a previous owner fitting a Milktek is possibly suggests passionate ownership and that expenditure wasn't an issue.
I have had two performance Audis with ceramic brakes, and my personal opinion of them is they are superb. Mine were not 'wooden' or 'dead', as has been suggested by a topic on Pistonheads, presumably written by someone who has no actual experience of ceramics, and they worked with extreme bite right from the off. Other benefits are no rusty discs, no accumulation of corrosive hot brake dust on the alloys and of course their appearance.
With a car like that, you would entrust it to a Audi Specialist, not to your local 'Kwik' tyre and service bay with some spotty faced urchin working on it, so worries about the care (or lack of care) of the ceramics are perhaps over exaggerated.
I love the V6, V8 and other big Audis - in fact the 3.0 TDi engined A6 and Allroad made that a tremendous vehicle as another alternative.
I have been fortunate to regularly drive many, if not all, Audi models over the last 16 years, and owning 8 variations of petrol models from the 1.8 Turbo, through to 4.2 V8 RS4's, base my thoughts on those experiences....
But my ultimate car has to be the RS6 Performance....
But going back to the S8 - if you want it, can afford it and run it - go for it. A tremendous vehicle

, and you'll enjoy it...!