How far from the horses will you be? Is this rodeo/barrel racing or show horses? I think the Canon 28-135mm IS or the 70-200 f/4L would be your best choices. It sounds like budget is a consideration so it might help if you told us how much you have to spend. Sometimes getting the job done is more important than actual lens quality. If that's the case, take a look at third-party lenses by Sigma, Tokina, and Tamron. But you will be sacrificing autofocus performance with third-party lenses and I think that's important for any kind of action.

As for shooting technique, there's a sports photography forum here. I don't think shooting horses in action would be much different from shooting mountain bikes or team sports. You should be able to get some good ideas there. And buy horse magazines and study the photos for angles and shooting styles. That will help a lot. If you have questions about how to achieve certain affects, we can help you figure out the techniques on the sports forum.