So I started developing something remarkably similar about 4 years ago (in Django, too!). Wrote up a business plan with monetization, p&l, etc. I brought the plan and prototype to a few seed folks for early funding before I admittedly lost interest and moved onto something else.
The hardest technical challenge I had was, knowing there would need to be a ton of recipes in the system to make it truly effective, I had to do an automated intake of recipes from many places. But ingredient normalization got in the way, even with really good regex/etl practices. For instance, a recipe says, "boneless skinless chicken breast".. another says, "skinless boneless chicken breast". Some list the # of breasts. Some list pounds. Some mean the breast is split, some don't. But in order for the nutrition info to be accurate, the normalization process had to be near perfect. I ended up "buying" the source code to "recipefox" a recipe parsing plugin for firefox (for $100 or something like that) which helped tremendously, but st