Hi,
I recently joined this forum ,having in August 09. purchased an 2004 RS6 Avant thought I would let you know what I looked for.
Obviously the cambelt is a major, the car I bought had just had it done at the cost of $3k, every 60k is required.
The front brake rotors had also recently been done $3k, new Pirellis $2,800 A service $1200 , also the shocks replaced $6k.
A lot of money can be spent quickly so make sure that most of the items have been done !
A full service record at the Audi dealer also is a must in my opinion to preserve resale value as well as ensuring you don't have any un forseen problems
The Rs6 is an incredible machine and I haven,t had any problems since I bought and now travelled 10k in the last 6 months. I looked at various cars like the Merc e55 AMG but they are generally dearer and aren't four wheel drive and in my opinion are no where near as good value as the RS6
As far as price is concerned I think being an import devalues it a little however with a good service record would still be Ok.
I second what Allan says. I've owned my 2003 RS6 for about 20 months (and done 45,000 km with it so its at 145,000 km). Make sure it has a service history, and consider it getting a pre-purchase from an Audi specialist. Make sure you go into it with your eyes open, and expect parts and servicing to be expensive. Get a mechanical warranty - it will help with some of the costs. It is a brilliant car, and I absolutely love mine to bits. It's just that everything costs more to do - e.g. thermostat failed. A $240 part except that you have to do a cam-belt disassembly to get to it - 7 hours labour needed, and the water pump gets done at the same time. Tyre pressure sensors are $350 a pop and they can fail from time to time.
They are very tuneable but beware the potential impacts on the transmission - torque converter and gearbox are known weak points with chipped cars, although reputable tuning outfits (eg MTM) have TCU software upgrades to help minimise the impact. I've gone with Bridgestone RE050A tyres at about $550 each, these seem to be wearing well although I don't think they have quite the wet weather grip of the Conti Sport Contacts.
Push the vendor on price - I paid $57K for a 2003 100,000km sedan 20 months ago, so try and squeeze them closer to $50K (even less if it is due any form of servicing as described by Allan). While the Avant usually goes for a bit more, and the one on trademe has the Navigation module, it is an import, and this counts against it.
Having said all that you are looking a buying an iconic car that outperforms almost everything else on the road even now, 7 years after it was released. What's more it's a very subtle car - most people don't even give it a glance, only those who know their cars recognise it for what it is. You fly under the radar screen (if you've got jammers

).
There is a BBC series called 'Last Chance to See' which goes around looking at endangered species. I think the RS6 is a bit in that category given the world situation with peak oil, greenhouse, and nanny state regulation of speed. Make the most of it while you can!
Oh, and another excellent forum is RS6.com, the US RS6 forum. Heaps of stuff there on tuning and service issues etc.
Good luck with your 'beast' hunt!