Later Porsche may be fitted with in car navigation from the factory. There are two systems, the earlier system called PCM from 1998 and the later from 2001 onwards called PCM2. Porsche communication Management controlled both the sattelite navigation and the car audio system. PCM came linked with Bose sound and optional phone facility.
On pre 2001 models there is not such a premium to pay for satellite navigation. On later and higher priced Porsche satelite navigation is more sought after and hence influences prices. This is especially apparent on Cayenne.
Although the factory in car navigation was a reasonably decent system when launched and the later PCM 2 was definitely an improvement. The earlier sat nav system could be hard to input whilst the ease of use of the latest in car navigation system such as Tom Tom leaves the Porsche system behind and offers much better value for money.
The major disadvantage to my mind with these built in systems is that the cd that is required to run the navigation is usually out of date, and to update to the latest cd is very expensive. Plus updates are slow to come through.
Although I am a lover of most things Porsche, the cheaper units such as Garmin and Tom Tom are easier to use, can be updated with the latest roads for free and programmed from the comfort of your desk rather than sat in the car. Plus you can lend the navigation us it rather than the Porsche when your partner, friend of offspring want to travel somewhere new!
Therefore, if you find the car you want just buy it irrespective of whether it has factory navigation.
Buying a satellite navigation system separately means you will also save money and have a wider choice of cars. You will be able to select the latest, best satellite navigation systems many of which incorporate speed camera / safety camera locations and some, radar detectors. This has got to be good news.
You are free then to change head your car audio head unit should you want to upgrade.
I personally like the Tom Tom and Garmin units, they are cheap, easy to use without instructions and accept post (zip) codes. The screen is clear night or day and for me its the best in car navigation system I have tried.