so many things I could say something about dont know where to start.. :-)
I'll start from the bottom.. lol ok how to turn on and off, on some dell docking stations they have a power button and a reset on them, this is ideal to extend into the cabin so you can use them from your seat.. as for auto on/off I did use a laptop for about 5 months.. I wired two relays in such away it auto powered the laptop on.. seemed to work well. The laptop then turned itself off after 10 mins of running on battery. You will not have the pcmcia adapter in the docking station, so you will need to remove this everytime you want to remove your laptop, or unplug the usb cable. I would buy a proper incar charger for your laptop, dont just wire it up to the 12v line.. not a good idea. I know www.battery.co.uk do them.. I'm sure you could source one from the net. If its the I7000, I think they have a round PSU connector, so you might be able to get a generic one.
As for playing DVD's and having to get out and change these every 2 hours.. nar.. just load divx films up on your harddrive and use mediacar to play em. (make sure correct codec's are installed)
not much to say about tv, as I havent gone down that road yet.
As for the sound output.. you might be quite suprised by the quality once its rigged upto your headunit/amp I used a dell.. it it sounded fine. I would be tempted to give it a go first, before spending extra cash which may only give you the same quality unless as you mentioned the nx audigy, but you dont want to spend that much money.
Also.. if you decided not to go for the usb dvd.. do you really need to go for the usb 2.0 at the moment ?
If money is to be spent anywhere I would be tempted to further upgrade your memory to at least 256mb if not 512.. I think the I7000 has a 8mb graphics card hasnt it, so doesnt share the memory.
thats about it really.. hope some of the info helps