Yes, WoW, yes! I love that game! But as to your questions...
1) It can be. It depends on how you look at it really. It's $15 a month, so, you might have to cut back on what you would typically spend $15 a month on (movie tickets, Starbucks, etc...). All the expansions are free except for the newest 2. Mists of Pandaria is around $10-$20 depending where you buy it from. Warlords of Draenor is going to be $50. You do not *have* to get the expansions because they will eventually become free anyway, but it does give you more stuff to do sooner if you do. Also: there are pets/mounts you can buy, but those are all optional. Not worth buying pets/mounts IMO, because they are expensive for a virtual companion. Oh, yeah, the base game is the same price as Panda. So base game, comes with free month, $10-$20, and it adds up with expansions. There is also a free starter edition that you can try out Wow for free up to level 20.
2) I got that mount too.

I think a paladin would fit the Hearthsteed the best. If not, then...maybe a mage? I rode that mount with my death knight which actually fit in good with my armor at the time, it doesn't fit anymore because my armor is too dark. Generally though, paladin, or a mage, would best fit the Hearthsteed. That's just my opinion though, there are other factors too, like race and armor.