http://www.amazon.com/Transgo-T84167..._cd_al_qh_dp_t - the first one is my review, since I wasn't having any problems you can ignore it.

Read through some reviews and you may be able to get an idea of lifespan. You may want to replace the trans filter while doing the shift kit as well. See what others say but that is a typical recommendation on here: drop transmission pan, new filter, shift kit install, reinstall pan, and replace lost transmission fluid.