I know you already said you didn't want a complete stereo replacement but looking at the prices of the Garmin 7200/7500 at being over $1k, I would seriously think about just installing a double din NAV/DVD head unit. For around the same price the additional benefits of going with an aftermarket head unit would be:
- Ease of installation. No need to fab up brackets to get your portable LCD GPS system mounted.
- No need to hack into the existing wiring to get your audio from your GPS system to play through your speakers. (another downside to this is that if you are listening to music or watching a dvd through your stereo system, the audio from your external GPS system will have to depend on it’s built in speaker which will be easily overpowered by your regular stereo speakers. If you have your GPS playing through your aux input on the factory stereo, you won’t be able to listen to music at the same time. I would much rather be able to listen to music while I drive on long road trips and then have the music auto fade down when the GPS needs to tell me to exit in a 1/4 mile. Remember, a good GPS system is one where you ideally would not be looking at the map on the screen much but be listening to the voice guidance instead while you keep your eyes on the road)
- Full upgraded audio controls. Having the built in eq on these new head units make such a big difference on stock speakers (since it sounds like you're keeping yours for now). The ability to control sending just mids and highs to your regular speakers and the bass to the subs makes the system sound like a totally different setup.
- iPod support. (this to me is the most important as all of my music and videos are managed via iTunes and my iPod.)
- BlueTooth wireless cell phone support
- You still get all of the features you were requesting including backup cam support and secondary monitor from your M&D DVD player
On top of that, since you already have the bezel, you can go with a cheaper plain 7” LCD monitor and use that as your 3rd monitor in the truck.
Anyways, it’s still ultimately up to you to go through with this project but keep in mind some the downsides of having an external GPS system.