Buy direct or from a bestbuy type of a place. Amazon may save you a few bucks but If its not an authorized dealer you will be screwed on any warranty work if needed.
As for upgrading the software, you have to have the unit plugged in so most of the time you need to take the computer to the car. And for the mac compatibility, well everyone knows somebody with a windows laptop and I am sure they will let you borrow it, I mean you own a Porsche for gods sake, how much of risk could you really be? You can always bribe them with a ride.
