The "G" Unit with TMC should allow your multifunction steering wheel volume and CD track controls to work with the the NAVI with no further setup except for maybe coding the Navi.
The TMC will also allow you to see the FIS information in the High Pixel Cluster if you ever deceide to get one.
Another little known feature of the TMC installation is that it activates the "Tail Back" feature of the NAVI so it will calculate detours for you if you ever get in a jam.
It cannot do this dynamically since we don't have "real" TMC here in the US but without the TMC you will not even have this option available on the Route options screen.
I have tried it and it works pretty well.
Another side effect is that with the TMC you also get 6 more station presets, (it works best if you connect the antenna to the TMC so it can actually pick up a signal).
So in total you would have:
6 FM1
6 FM2
6 AM1
6 AM2
6 TP1
6 TP2
for a grand total of 36 station presets.
Will you would need that many? Probably not but kind of cool to have if you travel extensively and want to keep station presets for more than 2 different areas, (the TP's are on the FM band).
Don't worry! Be happy!