Cards:
Playing Rappelz, the most important one to have is 1 or +2/+3 heal. The reason why Heal and not buff cards is that a heal card further reduces your cast time for heal, and heal is your single most important skill of course. It's expensive. Buff cards sell for around 1 million rupees for clerics.

Kahunas can also buff, and their cards are half the price of cleric cards. Some players generally rely on kahunas for higher level(rappelz rupees) buffs. If a kahuna doesn't have higher buffs, or there's not one in party, levele 5 buffs and we all still get by fine with that at high levels. Keep in mind that with the release of the new jobs soon, there will be much higher level buffs around from many of the new subclasses.

So don't fret too much about getting your buffs high level(Rappelz powerleveling). If you can, do so, but it's not that important. As far as priorities go, though, you should prioritize: Heal, Bless Weapon, Mental Amplify, because this helps your heals also - matk, Bless Armor helps the group take less damage.