Battery life depends on a lot of factors. Applications running, talk time, coverage, etc. Your battery life will differ from the next guy.
Take a look here: Maximum Battery Life - BlackBerryFAQ
Here is the latest official OS for the 8100: http://www.blackberryforums.com/gene...a-english.html
Follow the directions to the letter and you will fine.
Once your device is unlocked, it's unlocked. Upgrading/reloading the OS will not lock it.