Dvd
Several things to consider
A roof mounted screen is only a couple of feet from your face and therefore you really do NOT require a screen larger than about 7 or 8 inches. Get a screen with the best resolution, I finally settled on a clarion having seen a demo car, and the screen resolution is just awesome..not even close to the LR units. You need to get a really (and I mean REALLY) good installer/shop as this is the key to the entire setup and mounting a good unit.
A boot mounted dvd/mp3 player is a good option for a source, and can be hooked upto stereo. Another relatively easy way is via a fm modulator but its a huge effort to get rid of all the stray noice floating around...a few suppressors required. This can be your secondary means of sound.
The primary means would be just for rear seat passengers via an infrared set of headphones which automatically turn off (believe me you need this). Its nice for the kids to watch dvds and for you to listen to music....
I went with a sony playstation mounted under the passenger seat and also ran a spare set of leads to enable hooking up digital video camera etc.. Benefits of this are that I can reach down from the drivers seat and change dvds etc. Downside is I run the risk of twisting my neck
Btw. My beast is a P38a... not L322
MY 2c worth...less with inflation....