The new DVD V9.1 is available for $199. The only items updated is the mapping information (roads, streets, business info, etc.). You wont find any changes to the operating system.
The new disc, in comparison to your V3.1, is quite different. Thats over 6 years of changes!
Lastly, the installation is easy. It will only take 2 minutes to update after you install the new disc. The navigation computer is in the back near the spare tire. Simply slide the cover over the button and press for 2 seconds. The old disc will eject automatically. DO NOT insert the new disc first or you will damage it.
As far as the stereo is concerned you will be spending a ton to replace the radio, monitor, amplifier etc. Your climate control goes through the monitor in the dash too. I would suggest that you only replace the speakers, or amp and speakers.