In my experience it seems that Armor Pieces are VERY random, although you can watch for certain stats at the vendors. I have found that untill you can access Aleroth vendors and Your Class set in the Fortresses, its easiest to just go with what you loot and occasionally pick up at the vendor.
For a pure Mage my priority is on Magic, Ranged and Melee protection in that order. You get high magic protection just by investing in Intelligence.
This is just my observation and opinion so i am sure there are lots of others!
