I'm entirely new to modding vehicles of any sort, what sort of difference or gains would I get by going with a header + catback as opposed to just the catback?

And yes the price is REALLY attractive, the fine print on that particular one says the headers are ceramic not Stainless, but there are other similar auctions with stainless headers too. Might have to go this route based purely on bang for the buck. If I find another one with stainless headers, I'll keep an eye on feedback.