My personal experience with built-in, headrest-style entertainment systems has been negative. These things are usually added as an afterthought by manufacturers (whether console/headrest or ceiling mount) and lack sophistication. Haven't seen many rear seat entertainment systems that look well integrated with styling/functional cues. Adding your own "homebrew" setup will probably cost plenty and end up not looking all that "right".
I know you're not into the iPad option, but. . .
There's a great holder for the iPad that hangs off the front headrests. . . you get a much bigger screen, and so much more than just DVD playback. Here are a couple of links, just in case:
Don't like the color much.
Interesting design, questionable durability.
Interesting, to me. Think I might go this route, even thought it is most expensive.
For the cost of a custom aftermarket install, I can have 2 iPads with mounts, so my "customers" can watch their own content. I can take 'em out if I want, say, into the hotel (or tent!). Not only watch DVD, but play games, access internet, check email/Facebook, and, dare we say it, run "educational" apps.
My 2¢ worth. . . or as Gramps used to say: "Free advice, worth every penny".