Staff is perfectly fine, most people just like to use a skill with a Rage cost to quickly get Willpower back after spending it so they use a dagger/pistol and a catalyst instead. In the end the main thing that matters is just how much total "+x-y damage to Spells" you can get (not percentage damage, that's generally useless), whether it's on a staff or a catalyst. If you're not using a Rage spender, you just need to slap some Willpower Regeneration on your jewelry with Amethysts in support slot II.
As far as what you should build, that really just depends on what you're aiming for. The most popular mage builds at the top end just stack ailments and dps with Plagueburst and Infinity Blades, or abuse the infinite-cast Winter's Grasp bug (though I wouldn't recommend that one since it stresses the game and requires frequent client restarts to avoid intense lag, plus doesn't work until the skill is level 62 anyway), but when playing through the story most anything can work.
Personally, I love the Aether aesthetic and went with Annihilation, Infinity Blades and Anomaly as my dps skills, and went with the Time Weaver and Abyssal Shaper specializations (which coincidentally are used by the top end builds given their power), "Which Time Cannot Heal" from Time Weaver more than doubling my damage to things that are affected by Stasis and then "Occult Affliction" from Abyssal Shaper more than tripling my damage again when things are stacked up with Curse. I used the "Grievous Afflictions" node from Cabalist to inflict 2 ailments and slapped +Shadow damage to Spells so was stacking up both Stasis and Cursed at once and just watched the damage skyrocket, act bosses died in seconds.
