2004 is when 4F was released Link:-
Audi A6 - Wikipedia, the free encyclopedia
For example the injectors have been revised x times... not a recall as wear/issue happen after x miles...
The chain tensioner issue again is production/service regime/mileage dependant...
Above are the biggest to you could face...
Injectors DIY change approx.£1.3k.... @an indi garage £2k ish... @Audi £2.5 to £3k...
Chain rattle fix not DIY doable, engine and gearbox must be removed... @an indi garage £2k ish... @Audi £3 to £4k...
Next if an Auto (Tiptronic) then early ones need a software update and oil change to modify them when they start playing up...
Now lets get into software with modules... these cars rely on them heavily... get them done at a dealer is a real

...
I've only managed to get some updates done by dangling loyalty, servicing,etc and bouncing tech documents at them...
Yes many early A6 may well be good, yes there maybe happy owners but with age and mileage they will start to show issues which later ones will not have...
My VIN is 4 digits that how low it is, the rest is zeros... and trust me I wish I'd never got into what I have

...
Some tech docs to view to show you the cars complexity Link:-
Possible 'new' Owner - Page 3
You will really want to know the production date vs first registration date...
Audi dealer/customer services can tell you above...
2007 onwards in my research for documents,etc escapes a lot of the big £££ stuff...