didn't know anyone was still making a kit for these trucks. the price seems fair-ish to me, with the drop brackets and all.
as far as re-gearing figure out what you have now, Ford Ranger Rear Axles - The Ranger Station
gear ratio can be found on a tag on the rear end or through the code on your driver door info sticker. you probably have the 7.5 and depending on if manual or auto trans anywhere from 3.08s to 3.73s and you'd probably want to go with 4.10s which i believe is the lowest gear you can get in the 7.5