1. & 2. Average price for 1 person is about 800 CHF per month, but shared (student) house could go at about 400 at the lowest.
3. Cost of groceries will be the same as Zurich but eating out will be about 30% cheaper
4. This all depends on where you live and work/study. Live and work in St.Gallen and you'll not need a car. If you live in a village then you'll probably need the car. Commuting to Zurich then you'll have a tough choice since it sounds as if you are cost sensitive. Car ownership is expensive but so is an annual rail pass.