Count yourself lucky its not the V10. The V8 is generally reliable, however you're buying a 2016-2018 supercar, so it's been around. History is everything. If there's more paperwork than a Brexit export, then you're on the right lines. If the tyres are on the legal limit and brakes need a refresh, then be aware it will be expensive and more importantly question why? Simply saying RS6 seems to add £2k to everything. I'd want the car scanned first off with VCDS to see if there's any historical DTC errors logged. This is a performance car so oil changes every 5-8k miles. In terms of issues. Air intake valves can carbon up. You could treat it to a walnut blast. It's sensor heavy, so if original battery, factor in replacement. It needs coded to car and vented. If panoramic roof, check drain holes. Run mild detergent down them (exit above hinge on front door) and behind rear wheel arch liner. VAG have specific grease for sunroof. Also clean rubber seals. Assuming LED headlights. Potentially LED matrix if later car. Then check operation. Dipping light adjustment on start up is normal, but any further sporadic movent underway points to damaged or siezed height adjustment sensors. Servicing, diffs could use gear oil change. Depends on mileage. It's a ZF 6-speed transmission, but there will be a filter kit, so unless evidence of having it done, factor in. Cooling, apparently this thing has multiple radiators, fans and oil coolers. All are prone to wear and tear. If the car was remapped, be very wary.