you should really read all of the current posts and use the search function before you ask questions that have been answered many times before, it seems that many people here have short tolerances for that behaviour
In any case, I don't know what you're talking about when you refer to hooking your LCD up to your sound system.
You can get a GPS that plugs into the laptop, and just use the GPS whenever you want, it's not like it's a big hassle once the laptop is already on. Also you can take advantage of the suspend mode of your laptop for very quick bootup times.
be careful about the DVD drive, as most cables have a maximum length that isn't all that long. depending where you mount the laptop most likely an external drive would be better, but seriously, I have no idea why everyone is so gung-ho on putting DVD on their carputer. It's not like you're going to watch movies while you're in control of the car.
Without posting some pictures of your car, and telling us how big the components are, it's impossible for us to judge where you should put stuff.
Read around here and you'll get all the answers that you want to your questions.