Beware used 24-70mm lenses. I have had five of them (1 New), and none of them were very good. On a crop, they are unbalanced and difficult to hold still.
Be sure to try the lens out, and check it for the many problems like decentering that many of them have. Odds are that the owner would not even be aware of a problem like that.
A 17-55mm EF-s is the best choice for a crop body for general use.