For in car use? He will need a decend set of audio outputs to wire up to the cars sound system. Even the best laptop speakers would get overshadowed by the road noise.
Headphones would be the best idea if the in car DVD was for kids or passengers. If thats the case... most modern laptops will do the job.
At that price your looking at a 1.2 celeron from Dell. Other manufactures may be cheaper or offer more power. But Dell is good and 1.2Ghz is fine for DVD playback.
Main issue will be screen size!