been readin this forum for a while, have ya? then you know that there aren't any motorized touchscreens available yet, and the ones that have been "coming soon" are priced over $600.
And the hard drive thing has been covered so many times that when you searched, you came back with the phone book, so here's your freebie: Modern hard drives are not likely to fail while operating in a vehicle. That being said, laptop drives are even less likely to fail, so if at all possible go for a laptop drive.
MP3's can run fine on even a 266MHz processor, but for video playback 400MHz is kinda low. You didnt mention how much RAM you've got, or the make and model of video card so I cant in good faith tell you that you can't play videos. BTW, are these DVD's from disc or DivX files?