I am trying to figure out how much bandwidth I'm gonna need for a site selling body kits with 300+ products. It is a new site, so it has no reputation, but it will be the largest distributor of exterior import car parts in northern California. So how much? 30? 40? 50? More?

I would say under 20 gigs. I have a site that has 400 products and it has hundreds of visitors a day. It uses 15 gigs only. As long as you compress the images and everything you shouldn't really need that much bandwidth especially since you're new. As your site grows, just upgrade the plan.