Upgrade plan: what lenses for 5D3?

As it is today the price difference between the 5D3 house only, and the camera kitted with a 24-105 is just slightly more than what I will have to pay for an aftermarket 24-70 first generation. I very much want that extra speed, but my main priority is image quality and sharpness. From what I read I get the impression that the quality of of the old 24-70 lenses vary quite much, and that especially among the older ones there are many units that perform weakly on large apertures.

Would it be safer to buy a 24-105 kit lens I wonder, and could I expect to return it for another one if I should be unlucky and get a "soft" one?