Will,

I started with a 40D, then a used 1DMKIII and then a used 1DMKIV and now I have added a 1DX. The 1DX is an upgrade from the 1DMKIV especially in high ISO performance but I think for what you want to do a used 1DMKIV will perform very well and if you are successful then as your budget allows you can always add a 1DX later. By that point you might be able to get a good used 1DX. The 1DMKIV will certainly "hold it's own" for outdoor sports, the AF is fast and accurate, the frame rate is amazing and image quality is tops.... I kept mine.