Changing the turn signal light is not that difficult. The biggest problem is getting to the light. There is a rubber sheet in the wheel well that will allow you access to the headlight module. Just turn your wheels all the way to one side so that you have some clearance, remove the anchors in the rubber piece and swap out your bulb.
As for your roof noise, check out this thread:
C6 top still makes noise A lot of people around here have had the same issue. The E-clip mentioned on page 3 is what solved the problem for me.
As for the upgrade to the DVD navigation, there is always the aftermarket route. I do not know if you can upgrade to the factory nav uint.

Jason