i'm a PC, but i'll post what i know:
1) $1500 is a start of a very rough budget-- each install can be very different, so it could still be well above, or below that..(my first intel-atom based setup cost about $700, then i went through 3-$200 sound cards....)
2) i know that all of these are possible with a windows setup, but i don't believe that all of the software has a mac equivalent-- which is why some people run parallels, or other windows emulator.
last i heard: mac gps software is horrible, music is completely feasible with the FE's available, i don't think that any satellite boxes are have any software to work on mac yet, backup camera can be a feature of the screen you get, instead of integrating with the OS, video 'should' be the same as audio-- it should work with some FE's available, and internet should not be a problem as long as your wifi dongle supports mac...
3) the way i understand it, "to be different" is about the main advantage, overall, i think that there is a much larger base of products and help in the micro$oft camp here, but it is your choice in the end..
for FE's, last i read/ heard, there are not any new/current mac fe's around-- most of them are kind of old, but bluehawk solutions is working on a newer ipad/ipod/mac compatible fe-- but it has not been released yet--so it might be a little while.
it is personal preferance if this whole carpc is more trouble then it is worth-- to some, it is a nice hobby, to others, it keeps us busy...
just keep in mind that a carpc of any type will require maintenance at one time or another, so it can be more troublesome then a all-in-one solution like the avic.. but at the same time, a avic is less customizable then a car pc at the moment, though the guys over on the avic forums are working on it..(i know that they have gotten wifi to work on the avics..)
and you might have already seen it, but here is the general FAQ's: