Should we eat only organic once the baby comes? (breastfeeding)

Depends. The choice to eat only organic food is an individual one- depends upon your budget, availability of organic foods in your area. It is a good rule of thumb to eat a well varied diet. Some foods are grown with more pesticides than others- you can look at lists of foods that are recommended to be purchased pesticide free.
Avoid toxins now. If you are pregnant or plan to be, i would make some dietary changes. Avoid toxins and pesticides in your food, be mindful of what you are putting in your body that is nourishing a new little life. There are veggies and fruits that do not need be organic to be safe. Look up the "dirty dozen and the clean fifteen" list to make your choices. Also keep in mind meats and their sources.