Windows works ok on the mini but...
At the moment the ir remote dosen't work and the apples bluetooth drivers are abit shady, sometimes it doesn't work and you have to reboot to get it to work.
And sound can be a problem depending on how you planing to hookit up to your car.
don't think the 3G card will be a problem in osx or xp.
the biggest problem will be finding a decent gps program. the aren't any for osx and the selection for windows isn't that great either, the exist but the are more desktop applications and not very "carpc" friendly. unfortunately the best turn-by-turn navigation software is for pocketpc and smart phones.
Why do you need an extra dvdrw drive ?