My friend had an '04 RS6 for about a year (selling it about 7 months ago due to a move to the US).
A couple of things went wrong for him: the alternator gave up and required recovery back to the dealer, and as the engine bay of the RS6 is full of cylinders, turbos and intercoolers, it’s a front bumper-off job to change the alternator – cue a bill for £900.
Although he didn’t have it long enough to need a brake change, the discs on the RS6 are enormous (as are the tyres), so expect a fair chunk there.
Just before he sold it, the right-rear developed a ‘creak’ (which is a common RS6 complaint) to which the main dealer suggested that ‘around £1500’ would be needed to fix.
Don’t forget the premium fuel (which it loves at a rate of 12-15mpg on spirited drives).
Saying that though, it was absolute joy to drive, and he regularly commuted to Brussels in it; the relentless urge of the power delivery was incredibly addictive, and although the ride was stiff and pretty harsh, it wasn’t too bad.