Note that this has been edited to reflect the changes in Cataclysm / The Shattering, so the stat weighing is not final and a proper raid-balance of stats is not concrete.
Int provides you with spellpower and a larger mana pool, so it is naturally a strong stat.
Haste is also important, as it allows you to cast your spells faster, and to an extent, reduces the GCD (which makes a difference for instant cast spells). You get a good amount of haste passively through talents though (9% through Judgements of the Pure, 3% through Blazing Light, plus flat cast reductions on certain spells).
Crit is always nice to have, and it synnergizes nicely with some talents (Crit Holy Shocks reduce cast time on HL / DL, for example).
Mastery (at 80 and beyond) gives your heals a bonus shield equal to a % of the heal amount on your target. Shield effects have always been quite effective, and this more or less gives you a permanent Valan'yr effect to work with. I can see this being a very popular mastery, however we'll have to see how well it scales further on.
Spirit is now the go-to stat for mana regen as of The Shattering, and will continue to be so throughout Cataclysm. There may be a FEW items / enchants out there that provide MP5, but for the most part you're looking for spirit for your regen needs. It still remains to be seen just how much spirit is going to be needed to sustain throughout hardmode fights (especially with the ability to Judge Seal of Insight AND Divine Plea as decent mana regen abilities), but regardless, Spirit is the new regen stat.
As for leveling, just worry about Int, Crit, and to a lesser degree, Haste. Early on and leveling, a lot of healing will be done with Holy Shock and Word of Glory, which are both instant. Haste wont play a big role until later when you need to utilize your Flash of Light/Holy Light/Divine Light spells a bit more.
Best Answer
First, you have to complete the intial garrison quest chain, which ends with establishing your outpost and building barracks level 1. You right away receive the quest to upgrade it to level 2 (you need 200 garrison resources for that). This can be done within 1.5h maximum, starting in your capital (tested today in the morning).
After that, you are directly offered the quest to upgrade your garrison to level 3, but you need 2000 garrison resources for that step. Farming this amount of resources the conventional way takes a lot of time. An alternative to that is buying two Huge Ogre Caches from the auction house. Not cheap, but it does the trick.
A comment from my side: You have to decide yourself if it is worth the trouble, depending on what you want to do with your demon hunter. You can participate in all raids etc. without spending many days or 10k gold on your level 3 garrison. If you only want to level your new professions, constructing the respective buildings to level 2 might suffice until the start of the expansion.