Not too sure if the hood used for 24-105mm f/4L will affect the effectiveness of built-in flash or not. External flash will be of no issue.
For 17-40mm f/4L, built-in flash will be somewhat blocked by the hood though.
If you decide to go for 17-55mm, then sell off the 18-55mm since you won't be using it for sure.
The question here is how often do you need f/2.8 for your regular shoots? If your shoots are something which your kit lens can handle, why even bother to get f/2.8? Perhaps its all for the IQ, but other than that, you're better off saving it or spend it on 24-105mm f/4L.