This all depends on how much you wanna get your hands dirty. If you really wanna build a Car Computer or if you want a factory unit installed. I think that there is an on-star equipped system that does traffic updates. I know XM systems have live traffic updates.
Well, Both of those are subscription. In my knowledge, if you want live, you're paying SOMETHING for it. Some people use interesting ways to get cheap internet access in their cars to get live traffic updates.
as for 1. its hard to combine custom car-pc stuff with propriety manufacturer stuff. either buy it or build it.