How hard and expensive would it be if I would like to retrofit the Bose sound system (or the other higher quality one) on a 958 cayenne?
I'm looking to buy one, but the ones I find that are in my price range, don't include it.

 

While I can't answer your question directly, I surmise it will cost more than a top-notch aftermarket system. And Bose will sound worse.

If you want great sound and all you can find are non-Bose examples, consider yourself lucky. It's far easier to upgrade the non-Bose systems.

 

A good aftermarket system (eg Pioneer, Kenwood) will sound better, cost less and have have up-to-date media and interfaces. Bose marketing successes far exceed their real audio reputation. I'd also avoid in-dash nav.

 

So you say I should completely change the screen and the sound? Is it possible for a 958? Does anything exist that works good with the iPhone? Not only audio, but maps as well.

 

All I'm saying is that, all things being equal. you'll get a better aftermarket audio/media system for less $$ than an OEM 958 Bose system. I assume you'd be replacing the base system. Since the Bose system is a proprietary system that isn't compatible with other "generic" components; you'd need to replace the head, amp and all speakers to retrofit a "Bose system" per your OP. Unless you have access to a used 958 system, that's going a huge expense for a marginally better audio system. You may loose the small nav screen in the instruments but IMO, in-dash nav is a huge rip-off; less capability than a $99 Garmin or even phone-based nav.