Welcome to the Mac-Car forum! And I hope I understood everything you are asking about.
1) Navigation. Yes, your best bet would be windows on a mac. Not because of hardware (USB connections and such) but because of software (iGuidance vs. Garmin Mobile PC). The mac navigation software is not as good currently as Windows navigation software. So that is why you need windows on a mac. Not only must you get a GPS USB unit, but navigation software as well. Bear this in mind.
2) Front Camera. This can work two ways (I know of anyway): a USB camera connected to the mac, which can come on every time you turn on the Mac and activate an app; or a camera connected to your LCD monitor, which can come on every time the car itself turns on. With the unit connected to the monitor, it will always be on and you can toggle between that and the mac; you just cannot record. With the USB connection, it will record, and you can have it on in your background while handling other applications; it will just take up a USB port and some RAM (at least 1GB).
3) Engine information. There is a mac application called QCar which can check the status; it is in demo mode and there is a fee for getting the full thing. In windows, should you buy a car regulator to power the mac/screen/accesories, the software comes with it for free. And you can use with the car regulator to view engine stats. I personally don't use it, but it is a cool function to have.
4) Aux. Generally, all you will need is a connection between the Line/Out headphone port, and the aux port in the car. That's it. If you wish further control of volume, balance, treble and such, you will need an amplifier, and possibly an equalizer. From personal experience, the amp is a must if you do not already have it; the equalizer is just extra. But yes, you can connect a mac with the aux port.
5) Good USB Hub. This depends on what mac you use. If the mac you use has 5 USB ports, a hub is not really necessary. If it has less than that, then you might want to consider getting a hub that is car friendly. The MP3Car.com store does sell them for decent prices; check 'em out.
Hope all this helps, and best of luck to ya!
Also, what car are you using??