1: I believe the Life/Mana Leech skills only work for melee, but weapon enchant versions work for whatever weapon it is on.
2: Increased Damage/Magical Damage for weapons is essential. Armor does well with Melee/Ranged/Magic Protection and Life Line. Jewelry are all mediocre except Healing Aura, but all levels require a (very limited) Malachite Gem to put on. Static Charge Aura is the best otherwise, probably.
3: Set damage. Only thing that matters is your critical chance (weapon rate + death blow skill) and heightened reflexes bonus. Some enchantments may activate (like life/mana leech), but damage ones won't.