I think most everyone here has an aftermarket Nav system. I originally wanted the factory Nav system. (I wouldn't buy a car without it these days. I make my living on the road) I asked the dealer about it. They said that they could get a Hummer with factory Nav but that it had just come out and that it would take a while. They said the aftermarket units were better than factory (Of course... what else are they going to say?) I asked that they wire the unit so that I could program it on the fly and so my wife could watch movies while we are on the road.
I'm not sure if you can wire up a hard drive to the factory unless you went with a "phatbox" from
www.phatnoise.com or something similar. As far as playing DVDs... If its not coded in... You're not going to be able to. There are no controls for it, etc. You could try to see if the dealer will swap in an aftermarket head unit. It will be able to do everything you want.
Good luck and please post pics when you start/finish the project!