I did something similar with the original Galaxy Tab and I think you're on quite the right path. I think the new 7" galaxy tab is a great choice, especially as it supports usb host mode, allowing you to run an external drive to the glove box or elsewhere rather than having to take out the tablet to upload new music. In my case, I opted to use a single din head unit behind the galaxy tab. This allows for steering wheel control, an amp, and equalizer. Although introducing a whole new unit to the mix could add more variables for issues, the benefits outweigh the potential issues. If space in the dash is an issue, all the additional bits can be relocated to more hidden locations like the boot or under a seat.
Power is fairly easy if you buy a harness for your car. You can then splice in either a broken down DC adapter or even a cigarette lighter outlet for future swaps.
With enough planning, this is all pretty easy to implement and you'll be incredibly happy with the versatility of a custom android install. Actually, the most difficult part you'll run into will the fabricating a mount for the tablet.
Best of luck!!!