Hearthpwn

I try to be F2P player and I currently have enough dust (7000+) to craft one solid deck for legend climb. However, I can't decide among these options (or I should just wait more):

1) Secret Galakrond Rogue - The deck looks fun but I need to craft three legendaries (Shadow jeweler, Flik, Kronx). I maybe able to build Highlander Rogue too. 

2) Highlander Mage - My favorite class. But it seems too unreliable and can be easily defeated by aggro type. Still need to craft three legendaries (Prime, Malygos, Siamat).

3) Demon Hunter - Need to craft two legendaries. The DH aggro deck brought me Legend very easily (without those two legendaries). With these two legendaries, I think I can play every build of demon hunter.

I currently have spell mage and spell druid decks. I don't like both of them.

You can successfully play Galakrond Rogue without secret pack, because it's optional. Highlander Mage is meh at the moment. Demon Hunter may be nerfed again so I would wait before crafting new cards for him if you're short on dust.

For winning - Rogue
For RNG fun (or rage) - Mage
I wouldn't choose the DH because of nerfs

If you aren't set on a deck then don't waste your dust. Every time I come back to hearthstone I end up amassing a bunch of dust be DE'ing old cards, building a new deck from the current meta and then play for a month or whatever. When I come back the deck is no longer around and I end up DE'ing it for my next deck. If you are set on hitting legend every season (or even just attempting) then I would just pick a decent looking deck that seems fun and doesn't require a massive dust investment. I'd also prioritise decks with cards that won't be rotating out as fast.

Siamat is bad in this meta. Put a starscryer if ur going for mage.