Hi Ady & welcome

Welcome to Cayenne ownership, one of those will be on my list when I swap the Jeep out.
When I looked at one a few years ago the 3 things mentioned were: Water pipes, there s a plastic manifold which goes brittle with age & heat. The replacement is around a £1000 fitted & is aluminium. The drive shaft centre coupling rubber diaphragm breaks up. £700 ish or replace the middle coupling for £150ish & 2 hours labour. The final rare one is pistons breaking up which is terminal for the engine, diagnosed by a tick noise from the engine.
I run the South East London North Kent group of TIPEC details are here:-
http://tipec.net/region_southeastlondon.php