Welcome to my world of rotted old houses. I started as the carpentry foreman for the company I am with now that specializes in old and historic buildings.
Best advice I can give you without looking at it is to get all of the old stuff out of the way first. Do not try to use a level, it will make your brain explode. Cut a few sample pieces of fascia and set them on the ends of the runs with a string line connecting the tops. Check the middle with a sample block and a gague block to make sure your sofit will work. Trim and shim as needed before trying your fascia board. Tack your fascia board in a few places so it can be removed if needed to make sure it works.
Inside corners don’t need to be mitered at 45 butt joints will stay tighter. Also you want to make sure you end prime all your cuts. If you do not the wood will wick moisture and rot prematurely. Wood isn’t what it used to be. The bottom needs to be perfect, the top does not. You can usually suck the plywood down with screws to close any caps and the metal edge hides the rest. After doing high end interior trim for years took me a while to figure out how different roof trim really is.