Welcome to the forum Chris, you'll find the members on here are a friendly and helpful bunch :)
Great to see a fellow c5 owner on here! :) The 2.7 is a great engine and is worth a punt, plus they are getting quite rare now! It's worth checking the service history to see what work it's had done. Obviously listen out for any wistling from the turbos and make sure there's no major hesitation when driving as turbos can be very expensive to replace! Listen out for any knocking from the front of the car which could be things such as engine mounts, suspension components or worse case a driveshaft.
Is it auto or manual?
Also check for signs of rust on the front wings as this can be an expensive job to replace these.
In terms of electrics, make sure everything works! Ie the air con which can cost around £1000 as a worst case scenario (and is in the case with mine, hence why I don't have air con!)
How come is your current a6 dead/dying?
And have you got any pics? (I love to see fellow c5 owners cars!!)
Cheers
Steve