Seems to me all you'd have to do to take care of the obligations is require a High Concept (or even Trouble) along the lines of "Obligations of a Slayer", so that the GM can Compel it whenever it would be relevant.
As far as powers go, I'm definitely down with Inhuman Strength and Recovery. I'd allow Toughness as well, since Buffy can take some souped-up hits in the TV series that would flatten a normal person. As far as Inhuman Speed goes, I'd maybe downgrade it to a one-refresh "Inhuman Reflexes", that lets her dodge and gives her an Alertness bonus, without the fast running that the TV show SAID she had, but never really showed. Then something for the dreams and you're good to go. 8 refresh, total.