Nicki, the 6D is not the premier body for sport. It is limited in it's focus points and many people swear is never focuses unless you use center point. I have a 6D and I have no issues with outer focus points, but I don't shoot fast action. The other thing is the FPS of the 6D is not particularly speedy. Again, if just depends on your expectations. If you NEED high frames per second, then I would look at the 1DX or the 7D for those moments.
I have to say, I am more than happy with my 6D since acquiring it recently. I took all of the considerations that others had mentioned and for the price, I was more than happy with what I got. Just make sure you are realistic in what you expect it to do. You can at this point get a refurbed 5DII for about the same price as a refurbed 6D. It is up to you to decide which will work better for what you need it for. I shoot low light sports and the sensor on the 6D absolutely kills it in those conditions.