Welcome to the forums
I recommend reading for a few hours more, most of your questions have been asked!
Anyway, i'll try to answer some of them for you (nothing to do at work at the moment!)
1. The vga cable for my Xenarc display is about 6ft long. The USB cable is about another 4ft longer than that...why they didn't make them all 4ft longer i don't know!
2. Yeah, I had mine sitting in the glove box in my Ford Escort, as it was a temporary install, couldn't be bothered to run the cables to the back
3. Most DVD drives should be OK with a few bumps. Unless you are going over 2ft wide potholes at 60mph, most bumps are hardly registered inside the car. Unless of course you have rock solid suspension
4. DivX is a type of video compression. It normally allows you to get a nearly DVD quality movie into about 700-1400MB of space, rather than over 5gb on a DVD. Due to it's smaller file sizes (and still good quality - depending on who ripped it ) it has become a prime thing to trade on the intarweb.
5. Many people have had their computers in their cars all year round, some for 2-3 years, and not developed problems. Of course, this will depend greatly where you are, and your local weather conditions. I personally have not had any problems with mine here in the UK over the past couple of years