I recommend you look into Joe Jenkins and the Humanure Handbook
. If you want to be self sufficient then incinerating all that natural fertilizer you and your girlfriend produces each day at great cost seems crazy to me. If you follow the Humanure Handbook each adult will produce one cubic yard of high quality, safe fertilizer for your gardens and/or fruit trees.
Even with an incolet you will need to deal with your grey water, even in the winter time. In a northern climate, I found the cheapest and easiest solution to deal with grey water that I could get my local authorities to accept is a septic tank. If I ever decide to sell I can just hook up my toilet again and Im good to go.