The classical, as in the standard shoulder to head portraits are done around 85 to 135mm. You can of course do portraits as wide as 50, 35, 24mm or even wider but of course with that comes with distortion and that's where the art comes in.
What I find really useful is get a copy of vogue and flick through it and look at the adverts for clothes. They are normally done by photographers at the top of their game. It will give you ideas on poses, composition, lighting and if you can spot it, the focal length that was used.
I can't say I have ever found the urge need to get out a 100mm lens in order to do a headshot portrait if I had a 85mm in my hand. The 85mm is where I like it to be. In fact, I quite like the effect on 35 and 50mm too. But it really ought to depend what you want to do.
This is the key here, if you can help it, get the tools that do what you want to do, not let your tools limit what you can do.
The latter is a important skill to have when you are pushed into a corner, sometimes it can work to your advantage and get shots out of the box but generally speaking, the former is where you want to be.
I personally never understood the need for the 24-105/4.0. To me it is almost a glorified kit lens, it has kit lens aperture and it's not as sharp as the 24-70/2.8. People say it has IS but for that focal length, IS is not a must have, it's merely a nice to have feature. I'd take a stop of light every time too. The only only time I would thinking of getting one is if I am outside all day long shooting nothing but architectual shots on the street with the odd street shot in between. But to be honest, if I were doing that, I'd pick the 17 or 24mm TS/E over that too (if price is not a consideration) for the architecture and then something like a 50/1.8 even for the street. Although F/4 is bright enough in daylight, I just don't think it gives enough subject to background separation to make a photo really pop.
DP is right with regard to the difference between 17-24 and 50-105. And most of the time you can always move forward, stepping back is often much harder.
Given the choice, I'd get a 17-50/2.8 and a 85/1.8 for about £500 over a 24-105/f/4 all day long.