OEM sunroof on gli's are rare.

1) New cluster with taco will cost you under $100. Power window conversion $200ish + installation. Exhaust tip $50ish. Doing a CXi to GLi conversion is not worth the trouble imo

2) Id pay around 5-8k for a cxi/gli depending on its condition and what it comes with. Just do thorough check and spot the obvious.

3) CXi is tolerable, it wont have the same comforts as your M6 though. You might as well go straight for a GLi. The price between the two are relatively marginal, just gotta be patient and look around.