For the bodypiece you're free to choose pretty much anything you want. If you're going to be using pure mage, then by all means use armor. I would suggest using Royal Knight Armor (
http://wiki.mabinogiworld.com/view/Royal_Knight_Armor) if you can afford it. This is my personal recommendation.
The other option would be to use Karis or some other mana reduction piece for the set effect -10% mana cost. However you don't really need to use clothing bodypiece to achieve this, you may simply want to use Kari's shoes and a couple mana reduction enchants (
http://wiki.mabinogiworld.com/view/Other_Modifiers#Mana_Cost_Reduction). I personally do this, and once they fix transmutation talent (which adds mana reduction points) you can get mana reduction even easier.