Excellent feedback all around on page three. Thank you kindly, everyone.
So would that Sigma lens I originally posted be a decent choice for now? I don't need to make the decision ASAP, but would like to grab a telephoto lens by the end of the summer.
From what I've gathered from the veteran experience here, either wait for the STM 18-135mm or one of the Sigma 2.8's? Which one of these would suit me better, the fixed aperture or the ranged aperture? I know someone mentioned both above me but I'm kind of on the road at the moment (not driving!)
With the 17-40, if you start shooting at f/2.8 and 17mm and then zoom to 40mm (or much of anything inbetween), your exposure will change from f/2.8 to f/4 and the image will now be underexposed by a stop. Your only choice would be to change exposure while you zoom (not going to happen in a single video shot) or to limit yourself to f/4 at the wide end.
With the 17-50, you can start shooting at f/2.8 and it'll stay there no matter what focal length you use.
I imagine there might still be T-stop changes over the focal length range, but it's going to be a while before you're going to care about that sort of thing, and a loooooooooong while before you'll be able to afford gear that deals with it.