If you purchase the game digitally, then you must download the game. Regardless of which expansions you purchase, the entire game client will be downloaded (your account type will restrict which areas you can visit, what level you can reach etc).
The entire game weighs in at around 25GB (as indicated by a Blizzard Support Forum Agent here: http://us.battle.net/wow/en/forum/topic/8197179898)
If you don't want to download the entire game, you can buy it physically from retail stores (online or high street), and install from the discs. However, due to the nature of the game, you will need to download patches which will still be in excess of a couple of gigabytes of data.
As far as I know, each WoW game comes with free trial periods of game time, but you will need to set up a subscription in order to keep playing.
And finally, not to put too fine a point on it, but if you are aware that a question has already been asked, you shouldn't ask it again. If none of the answers in the existing question are helpful to you, you can comment on them and request more information.