We have a 2005 4.5 v8; i was warned off the turbos until the 2008 model (too many leaks from the bearings) and on a few test drives noticed full pressure was not achieved as only one turbo working.
On the older models seen sagging roof linings, rear lid springs/hinges need replacing on the tailgate, eats brake pads and discs - huge stopping power, excellent braking.
I have driven 5.7v8 hemi jeeps, 4.0 Toyota Prado and Landcruiser, Nissan Pathfinders and Range Rover and Discovery - the only thing comparable to the Cayenne was the Range Rover; but the handling of the Cayenne i found superior. For a big car it is an excellent drive. For every day use my wife now prefers driving it to our 996 C4S.
We have had the fuel evaporation pipe replaced, and not much else other than the routine servicing. Although it seems the fader / dimmer switch on the dash lights is intermittently failing.!
Enjoy - the 2008 4.8s was very similar - but with more power and a sports button!