It's an absolute minefield. It's a very good advert as LeMson says but there are so many things that could be wrong and could land you with a huge bill. Gearbox, turbos, intercoolers, suspension, etc.
I would advise before purchasing that you have a reputable RS specialist give it a thorough health check. If the seller isn't willing then walk away.
I did, as Fox says, have a C5 RS6 for a year and it did cost a fortune to run 'properly'. No matter how much you spend there's always another bill around the corner. Be ready for this and it will be a very enjoyable ownership experience.