Do MTG Core sets contain new (never before seen) cards? How does this compare with expansion sets?
Core sets do contain new cards, but at a lower ratio than typical expansions. This article indicates that about half (112/249) the cards in Magic 2014 were new, while the rest were reprints. I would guess that the ratio will be about the same for Magic 2015.

By comparison, in Theros, out of 249 cards, there were 12 reprints, and 5 functional reprints (nearly identical functionality, but a different card name).