my advice,
do not start playing wow .
you haven't calculate the price of purchasing the expansion cdkeys, plus catac's.

on the contrary, you should ask anyone here for a trial account (from introduce a friend progrom) and see whether you like the game or not.
The trial account however, can only be played till level 20 with restrictions.

And yes, your characters will still exist when you upgrade your account.
you need to be at the "Playable" stage to be able to play.

offgamers.com is quite popular
you should check it out

you can actually buy the cataclysm cdkey (digital copy) or the cataclysm pack from offgamers.

are you serious? you play wow as a way to save money?