If you do it, please buy an extended warranty.
These are pretty solid IMO but from my experience, when things start to go they get real expensive real fast. Look out for air suspension (shocks and pump), suspension level sensors, headlight range sensors, tire pressure sensors, all manner of electronics. Check the electric engine mounts, might as well have the front control arm bushings checked out too. If the air ride goes out, you can always retrofit coilovers or just simple springs/shocks onto the car. Much cheaper than new bags/pump/control unit all around.
Also these cars have the chain-driven 4.2. IIRC the same one from the B6 S4, which had issues with chain guides, AC compressors and water pumps. I can't recall older A8s at our dealer having those same issues, but it's possible.
Recalls: Ignition coils if they haven't been done yet, also some had an issue with the driver's air bag cover and needed replacing.
Seriously these are awesome cars but I wouldn't personally buy one without an extended warranty, even coming from an out-of-warranty E46 M3 like I own. Not all of those things fail on these A8s, but when they do they're pricey.