There are loads of options nowadays to choose from.
Choose wisely, as "you only fit once" if your chosen monitor proves to be unreliable and offer a poor image... then you may be stuck with them if they have been "cut" into your headrest.
When buying a monitor choose one which offers a "6 oclock viewing angle" when buying a dvd, again choose one which is multi region, the latest stuff allows you to also play Mpeg4/Divx, may be worth considering, also has SD/USB readers etc..
As for the installation, you can get 6.5" monitor in your headrest and utilise the hollow shaft's for the cabling...
Consider Digital TV tuner, as they are widely available now for the automotive... look out for diversity so it still works well whilst on the move....