Insulating attic ceiling and looking for synthetic t&g ceiling boards
The most important thing to know is you still need air movement in the cold space. So air moving from soffets into air vents etc.  This will stop mold. Then because of the overwhelming exposure, you will need lots of insulation. Check your local building codes. Where I'm from, it's something like R-28 with vapor barrier, and then whatever type of drywall. 

Secondly, if you're making this a habitable zone, you need to have a way to heat/cool this space as it's now part of your dwelling.