Hi Rhys,
You are right there are many brands on the market that are selling DVD systems for cars, be wary of cheap ones due to the quality of the laser mechanism and overall quality, also be wary if you plan to buy one from OS as the zoning may not be correct for Australia, although many cheap brands are multi zone as they don't care about the legalities. Most DVD players are just that and similar to home ones where the need to plug in to an amplifier or TV for sound, this can be done in a car by just going direct into a normal car amp. In saying this Clarion have a unit that is a CD/DVD, AM/FM radio combo unit (one of very few on the market) this unit has a built in amplifier and can drive speakers directly of the headunit, another feature of this unit is it has a 2nd zone feature which allows people in the back to have headphones on listening to another music source while you listen to the radio or CD in the cabin. Clarion also have combo units incorporating 6.5" pop out screens for dash mount. In relation to splitting multiply screens, this is possible by using the a video splitter box which most top companies will be able to sell. For more details on the Clarion products checkout
www.clarion.com.au. The only reason I talk about Clarion is that is what I specify we use on our boats and if it good enough for $2 million boats it's good enough for most cars!! Hope this helps.
P.S The legalities of a screen on a car is any screen visable from the driving position must not play moving images whilst the car is underway, there is a wire on the DVD player or screen that "must" (used loosely) be connected to the hand brake.
Cheers
Greig