RE: Your drywall ceiling - You may not have to remove it. Then again, it may be a prudent thing to do. If the drywall is in good shape and it's been out there for years, then you can leave well enough along. Or, you can take your chances in seeing what it's covering up, and go from there. At worst, you could always hang 1/4" outdoor-rated plywood in place of the drywall (using screws) to cover up any 'bad news' that you might discover. If it turns out that there's wood up there, I'd consider cleaning it up and at least Priming it to kinda protect the wood. (I'm wondering if that was the thinking when the drywall was put up, that's all.) In doing this, you won't have wood rotting up there under your new facia.
If you have more ?'s, post back. My best to ya and hope this helps.