I have worked as a ghost writer on more than forty novels, including plenty in various sub genres of fantasy and sci fi. I have several published fantasy novels of my own as Stuart Sharp. I've also done editing work before, including almost complete re-writes. Since you're not in a hurry, I've gone with a couple of months to get this done, to give us the time to go through it properly and decide where it needs to go.
My basic approach to something like this would be to start with a read through and then talk to you to establish what needs doing, probably draw up an outline, and then go through the rewrite chapter by chapter, sending them as they're done. Obviously, at the end, I'd want to go through and tighten things, before I handed over a finished piece.