I haven't tried to shoot horseshows with my digital camera - the cameras replaced the horses. However, I would think that the DA 50-200 with the K100Dwould work very well. I'm not sure you would really need a 300 lens, unless you are talking about top shows and will be a long way away from the action. I don't think I even owned anything longer than 135mm for my film camera when I was showing (mostly local shows).
The one thing that might get me to consider waiting for the (not yet released) K10D over getting the K100D is the weatherproofing/seals on the new camera. I haven't had any problem with the K100D, but I remember how dusty it could be when I was showing horses in Texas. Its not an issue that would stop me from buying the K100D, just something that might get me to buy the K10D over the K100D, all other things being equal.