I wouldn't even touch a Nikon AF-I Lens like the 300mm f/2.8 as you mentioned, the AF-I motors are highly unreliable and cannot be fixed or replaced I looked into this myself last year and Nikon NPS & Fixation told me there is no way to fix them unless you get another of the same lens and mix parts across. If I was you save your money and get some better glass, the Sigma 300mm is okay but slow focus at times, I was in the same position as you and ended up saving my money and getting a more modern Nikon 400mm f/2.8 which is the ideal length for sports IMHO.