You don't mention where your from so it's hard to tell how much insulation you need. Are the walls 2x4 or 2x6? I wouldn't count on using the old sheating, it will be alot of headaches to remove and re attach it without damaging it. Just do yourself a favor and use new sheating, you siad it was only the back of the house so the price shouldn't be huge and you should save the extra cost by saving time by not having to be carefull taking it off. Also depending on where you are and what insulation you use in the walls you shouldn't need the extra insulation board on the outside. I don't use felt either, I use a tyvek material. Sounds like you might have jumped into something without really knowing what needs to be done.