First of all, you need to head over to the FAQ Emporium and do lots of reading. You're going to want to read up on the isues that are specific to laptops in a car, as there are specific issues you're going to have to contend with.
2. No HDD is shockproof. Laptop HDDs are better protected from shock by design, however.
3. It might if you hit a big bump while it's playing.
4. A powered USB hub needs power, but that's the only issue I know if.
5. Play a DVD on the laptop before you put it in the car to find out.
6. Again, try it outside of the car to test. There could be issues, but you're not going to know until you try.