The 28-135 isn't a bad option for you but just keepin mind that it isn't a very wide lens on your XTi. Sometimes it's nice to have something better at the wide end for your 2 year old (when your in close) but the 135mm at the other end will be nice for your horse pictures if you want to zoom in a little tighter.


Another option you may want to consider is the 17-85 IS lens if your willing to sacrifice a little on the long end and keep the wide angle about the same as your used to right now with your 18-55. The price really isn't that much different from the 28-135. If your willing to spend a little bit more, then the 18-200 might be a consideration as well. Optically, the other two lenses are probably a little better than the 18-200 but the versatility that the 18-200 will give you may make that up for you.


As far as buying used that is often fine if your allowed to really check out the lens first. E-bay I think relies a little more on faith. You can look at the refurbished lens at Adorama, which aren't bad deals. Or if your in the U.S. and near a metro area that has a Craigslist, you can at least see the lens up close before buying it.


Good luck!