Grocery store is fine. You want organic if possible. Not a deal breaker if they're not though.
Minimum of four quarts/liters of juice. It's a feast, not a fast. Four quarts PLUS water during the day. You want to supply your body with enough calories to live on plus you want to flush the system out.
Don't make the green juice all in one. Spread the recommended amount of greens through out all of your juices. The fruit masks the greens very well. You should enjoy them for the most part, not feel like it's something you have to suffer through.
I'm not sure how your dirty roomate will effect your healing? What do you mean?