Welcome to my world, I h ave owned 3 in-dash nav units so far.

The Alpine is the best and it looks to me like with a bit of $$ you could loose the lower pocket drop the Hvac controls down and have room for 3 Din sized units (I had the same problem and was quoted 200 by altman on www.mp3car.com to do that work or 500 at the local stereo shop (altiman does very impressive work check it out)

Alpine makes a stand alone display for there nav/gps system as does pioneer and kenwood. I would stay away from the audiovox stuff. or you could get a all in one unit like the pioneer avic n1 or N2 built in touch screen nav/gps, odbii interface etc. if you have questions let me know I have owned them all.