In the very beginning if you take it easy you can start without additional food. Once you get into the groove you will base your food intake based on your recovery ability (ie, if you're starting to fail squats before bodyweight is on the bar, you're not eating enough).
On those easy sets your heart rate will jump a little. You'll puff a bit, but it won't be brutal... till later. On my PR squats I generally have to sit the F down or risk falling down. But you'll be used to it by then - that's part of the effect of Linear Progression; acclimatizing your body to slightly higher physical stress slowly and constantly.
In the beginning, SS is perfect for anybody that wants to get stronger. Your goals dictate what happens after SS.