From what I've read, stay away from sunroofs. The panoramic sunroof has loads of issues, the regular sunroof sometimes could leak but it's an easy fix. If it has a sunroof and it rained recently, check the floor mats for moisture.
Do you know which engine it has?
And as for an aftermarket stereo, it's not bad and can be done. But can get expensive because of Onstar, if you have it, and the Driver Information Center (DIC) They make a kit to take care of the DIC but it only allows you to run a single din stereo, unless you mount that kit somewhere else, then you can mount a full screen stereo. But it can get expensive.