What do you pay attention to with a used Top A8?

Hello, everybody.
I play with the thought of changing my previous house brand after 17 years, and to get an A8/S8, the superlimosine par excellence, I would say. I don't have a specific model year in mind yet, but it is supposed to be a former top model in any case. I would like to get an overview of the model history, which I still lack completely at the moment. Were there any outstanding milestones in development, or any weaknesses in certain years of construction? Has the Quattro drive been changed/improved over the years? And the engine? What innovations and when?

I'm most interested in an S8 - on paper at least. What is the difference between the S8 and the A8 4.2 Quattro? Sports seats? Chassis? Brakes? Equipment?

What about the Navi system? Always based on DVD, and if not, when?

Topic Consumption/Requirement: regarding consumption I am quite spoiled, with my current 200-260 hp cars I need with a restrained driving mode partly just under 8 l/100 km (Benzin mindfully). With fast driving (but of course not lead foot) I hardly need more than 9.5 l/100km. This will hardly be possible with a 4.2 l engine, but at least I would hope for a comparable range of about 700 km. With a tank capacity of 90 l, this corresponds to about 12 l/100 km. Is this realistic for fast, forward looking, intelligent driving on long distances?

Are there any vulnerabilities in the car? What is the best way to recognize a beaten specimen? Oil consumption? Drive shafts?

Many questions... I hope there is at least one answer to some things, and maybe I can soon join the circle of A8 riders and share my experiences here...