Mounting kits for portable DVD players exist and can be found on many retail websites. It's possible to mount the DVD players from the ceiling, the arm rest or the front seats.
Theoretically indefinitely. The disc is only spun only when it's being read/accessed, so no damage can be done to the player from this side. THERE IS, however, a possibility, that an inserted disc...
It is difficult to say which one is the best as there are a lot of model out there to suit for different purposes. Some are used for their kids while long drive, some are using it while they are on...