I use $50 per day per person as a guideline for food and snacks. Mind you, I have two teenage boys who would eat the hind leg off a donkey! I found that figure to be generous on our last trip, and came home with some cash in my pocket. We usually do one counter service meal per day, and one sit-down restaurant per day. We do not, however, plan on any character meals which would add to the expense quite quickly. Nor do we pack any food to bring with us. $50 per day covered everything but nick-knacks.
PS's are recommended for the major restaurants. On our last trip (end of January), although I'd made PS's, they really weren't necessary. The crowds are pretty low that time of year. I also found it annoying to think that I had to be in a certain place at a certain time. You loose a certain amount of flexibility. If I was travelling the same time again, I would skip the PS's and go where I felt like, whenever I felt like it.
I am a planner - my two trips to Disney, I had every day scheduled. I wouldn't do that again. I thought if I'd planned everything, I could see everything. Well, you can't! Just realize that you will have to make repeated trips to take in everything, so do some research and figure out what is a "must do" on your first trip. Remember to relax and enjoy the vacation - don't get caught up in the "commando-style" touring!