17-40L is a good call ...has a broad usable focal range on a crop body.
Yes and no: it's ok on crop, light & good flare control - though the 17-40L is better on ff plus it's not sharp esp. at open aperture in comparison to the ef-s lenses. I'd only get the 17-40L for crop if upgrading/dual-using to/with ff is planned, or if weather sealing is important - in the latter category the 17-40L shines as an internal zoom.
If your budget allows, consider the 16-35II f/2.8 instead of 17-40 f/4, you will then have two fast lenses. But the f4 lens is no slouch and both are great walk arounds for crops.
Here I have to disagree - since there is the Tokina 11-16 for crop imho the 16-35L is complete overkill - constructing an uwa zoom for ff is much harder esp. if the corners have to be sharp, that's why the 16-35L is so expensive. On crop the mirror is smaller, the ef-s lens is nearer to the sensor and a good performing f2.8 uwa is cheaper to produce. Next to that, the 16-35L is heavy, and you're not even using part of the glass.
Disclaimer: I'd gladly switch my 17-40L for a 16-35L for free, but only because I'll get a 6d sooner or later.