Ok, first off you can basically ignore auto-attack and delay. Those aren't really useful for mages (and tbh aren't things most people care about even for melee). The main thing you want to look at for a weapon is the Weapon Damage stat. This affects the effectiveness of ALL your actions, and is one reason why your weapon is the most important thing one can upgrade.

After that, comes your primary stats. The only ones you'll care about as a healer are MND (increases effectiveness of spells), and VIT (Increases HP). The VIT that comes standard on non-accessory gear should be sufficient, but it is worth noting that if gear does not have MND on it, it's not made for you and will be quite ineffective. Technically, you'll care about INT for the damage of spells like Stone, but in practice you just use Cleric's Stance to swap your INT and MND when you want to deal damage, then swap back to heal. Also technically Piety is a primary stat, but in practice it's more of a secondary stat.

Then there are your secondary stats. They're all useful in varying degrees except for skill speed (which shouldn't be on gear made for healers anyway). Some ARE more useful than others, but until you hit endgame you'll get more of a benefit just from boosting weapon damage and MND by increasing item level, rather than holding onto a piece for 20 levels just because it has spell speed and the upgrades don't.

Finally, you asked about Potency. Potency is a way of comparing various skills against each other. It can just be used as-is to get a rather close approximation of the skill's effectiveness, for instance one can assume a 200 potency attack is twice as effective as a 100 potency one. When it comes to damage/heal over time effects like Aero, just take the duration, divide it by 3 (they work every 3 seconds), and multiply that by the potency to get the TRUE potency (Also spell speed affects these, or skill speed for physical jobs, so they can wind up a fair bit more effective depending on stats)