You'll need a lens which will be at least 200mm at the long end of a FX camera, equivalent to 300mm on the DX sensor. The lens speed needs to be f4, even better @ f2.8. If you get into low light; late day, evening, you'll need the advantage of f2.8 for stopping the action and highlighting the subject. The fastest shutter speed is 1/4000. You're probably looking at a used 80-200 f2.8, and that's going to blow your $500 budget. You might get by with a 70-300, but the minimum apertures are going to be in the f4-f5.6 area.