There are many options available as far as DVD players go, but don't get "suckered" into the smaller screens just because they are cheaper. Ideally if you had a laptop computer it is cheaper to get the DVD rom drive added, than a stand alone portable dvd.
In any case I digress, you want to get one with at least a 6.8" screen...several available with 5.8", but frankly that's a little small. Brands to stick with are Toshiba and Panasonic.
Your third row seating passengers will have a bit of a problem viewing, and you will have to consider how you 1) power the unit..only audiovox and aiwa make units with cig lighter adapters...so you'll most likely need a power inverter (
www.jandr.com) will be able to help you with both the palyers and the inverter, unless your car is equipped already. 2) how are your passengers going to listen...via headphones (usually only 2 outputs, but very inexpensive splitters from Radio Shack will solve that) Not sure what type of vehicle you are setting it up in (SUV or MV), but they (J&R) also carry a bag/strap device to suspend the player across the 2 front seats.
I had it in my wife's Lexus RX300 (which doesn't have a 3rd row) and set it up on a sturdy cardboard box in between the front seat armrests...mainly because I didn't want my DS to be tilting his head down (from his car seat elevation) to the relatively low center console location.
Let me know if you want more info...I can go on about electronics (i.e. expensive toys) for hours.