Along with some informational stuff about the state of the game...
Currently, to gain legendary rank in Hearthstone, your deck has to be able to contend with some of the more powerful, yet straightforward deck-types to make it past the rank 1-25 and into the orange legend's house. Once you hit rank 5, the win-streak bonus stars go away, and you essentially need from that point on, to have ~30 more wins than losses overall.
The current powerful decks that plague the high rankings are Warlock rush, Warlock giant and Rogue miracle. Other decks are also there, but are nowhere near as prevalent as these types.
If your deck can consistently pull out a >50% win-rate against these types of decks, then you'll eventually make Legendary if you keep at it, the experience gained from a loss against one of these decks will help you the next time, and eventually you're able to perfect your game to the extent where you either know you're going to lose, and need to adjust/change your deck, or know how to win, and can chalk losses up to things like "bad starts" or "crazy RNG" that evens out over time.
It's a fairly cheap deck, only one legendary and three epic cards - so most people should be able to afford it, or at least something similar. Though, if you're really short on card options, a warlock rush deck might be a lot cheaper and off the top of my head doesn't require any legendary/epic cards - Leeroy is optional there.