If you are not going to get a super wide, then the 17-55 swings very strongly into contention. The 24-105 will just not be wide enough on a crop body, I think that is pretty safe to say. Don't be concerned about the build quality on the 17-55 2.8 IS - it is a high grade lens in every respect, as you would expect for the price. Get to handle one.
The ideal three Canon lens kit for a 50D is: 10-22, 17-55, 70-300 IS, and everyone one of those lenses is the best in its class. It's an exceptionally good set and is the mainstay of my outfit (even though I have swapped out the 70-300 and gone for a two lens option there of the 70-200L and 100-400L).
Of course there are plenty of alternatives, but that is the trio Canon intended to work as a set and if you want to devaite from it that's fine, but think twice about why. The gaps and overlaps are also fine - you need overlap with wide angles to save you swapping lenses too much on walkabout shooting, but it's much less necessary at the longer end and even a gap from 55-70mm is easily worked around. This is certainly the way I like to work anyway.