I guess at this point I'm just playing devils advocate here, but what luxury features does the Cayenne or G500 have that you desire? Where do you intend to take the vehicle? Have you considered Discoveries, Range Rovers, late model Trooper/Acura SLX's, Infiniti QX4's? All those vehicles can be chocked full of luxuries depending on the trim level, would cost you a fraction of a Cayenne or G500 and have a larger support network.
My biggest worry, particularly with the Cayenne, would be drivetrain damage. Yes, you are not going to be taking it rock crawling, but you hit a rut at 30 mph on a desert road and you're in a heap of trouble. Nearest town is Boondocks, Nevada and the local parts store ain't gonna have no Porsche parts, or even an interchange part. So now you're either hobbling it to the next place that has a Porsche dealer, potentially causing more damage, or getting a tow truck at a substantial cost. Then the part is probably dealer only, and dealer install only so now you're out several more thousands of dollars. I mean, I hope this doesn't happen, but these are things to consider.
Additionally, even if you can pick up a Cayenne for say, $30k or so, modifying it as an expedition vehicle is going to even further drag the resale value down because there are probably only a handful of people in the world that want to have one set up like that, so you'd likely have to sell it at a loss down the line. It's not like a Land Cruiser or D110, where a nicely setup rig can command a premium because people
want those vehicles setup that way.
Again, don't call me a hater, these are just questions that I would have. I just hope that you are not traveling down a path that you might regret later. If you do move forward with a Cayenne build I will definitely be interested in seeing the progression and even more interested to see it on the trail. Perhaps when you get it done we can meet up over a campfire and I'll sip my Jack from a tin cup while you enjoy your 12 year Tullamore Dew from an elk hide bound titanium flask.