All three of the Blizzard Store mounts (the Heart of the Aspects, Winged Guardian, and Celestial Steed) work from Level 20 and 75 (slow ground) Riding Skill. They will not, however, allow the user to fly until the proper riding skills (225+ Riding, Flight Master's License/Cold Weather Flying) have been purchased (based on zone).
(Source: Wowpedia, Wowhead)
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.
Best Answer
"Scale" in this case refers to "Scaling with stats" -- or, put more simply for this case, "How the Holy Paladin's ability to heal improves for a given increase in a specific stat" (Which are acquired from stronger gear, or temporary boosts, such as are being discussed in the guide).
While I can't speak to the specifics about healing as a Holy Paladin, the implication is that the effect of this ability (the bonus Intellect) is less useful to a Paladin than another stat -- such as Critical rating or Mastery.
Unfortunately, there is no general way to determine this, as each class is different, and often that there are multiple potential interactions between spells, abilities, buffs, etc. to truly account for a "Standard" situation.
That doesn't mean people don't give it a try, and the folks over on Icy-Veins, for instance, have gone after the problem with more spreadsheets than you can swing an arcanite reaper at.