I forgot to mention the portable DVD player needs to have a video input. Most people prefer the Audiovox VBP800 portable DVD player but they are hard to come by but if you google around you can find guides that cover different dvd players.

As for the voltage you have two ways to do it. Use a voltage regulator and increase the 5 volt lead to 9 volts or go even cheaper and toss on some resistors. Now just so you know some LCDs with this configuration will make the display real bright or off color but some good old fashion potentiometers can fix that. Just run the 9volt to the lead that controls brightness, contrast, tint and such.