I just used my 10-18 EF-S lens and it is an inexpensive light option for you.
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But I have the lenses (300 f/2.8 and 400 f/2.8 ) to shoot sports. My recommendation to you, based upon your lenses, would be to get the 1Dx. It fits both of what you need: low-light and sports. My worry is that you are missing a long lens now should you decide to amp up your sports shooting and do field sports. Otherwise, you would have sufficient lenses for indoors and the ISO performance, as well as superior tracking abilities. The 1D4 was not nearly as good as the 1Dx at ISO 6400 and we all know that can be a common ISO in tough lighting. I have a few basketball galleries, all shot at ISO 5000 on a 1Dx and every file is perfectly clean after applying only a 40 in NR setting LR. The 1D4 took more work, if it could be done. The 1Dx's AF system is noticeable "snappier" than the 1D4, and the 1D4 is in my opinion more accurate than the 5D3.
After reading his glass options, I'm thinking a single IV and a round of upgrades to his 24-70 and 70-200 IS, plus a 6D? Having 2 identical bodies makes moving between them much easier, but it seems that you're all over the board, and sinking your entire budget into a single X isn't going to solve all your issues at once.
Not that spending money isn't fun, but hitting these upgrades now would make adding a 1Dx in a year just that much stronger overall.