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Light armor is typically the best for most PvE magicka builds.
With that said, if your Dragonknight is a low level, still leveling, I recommend a 3/2/2 setup of light/medium/heavy just so you can unlock the passives for all three armor trees.
Inferno staff is a nice compliment to the Dragonknight fire damage boosts. It's also the staff that does the most damage in the game, at least for single target. (Lightning staff has some AoE capabilities due to passives, and ice staff has some control/shielding boosts from those same passives).
Light armor is the most important; however, you should level all three as you'll ideally be wearing 5 light, 1 heavy, and 1 medium at end game. This will increase all three resource pools once you have the undaunted passives plus you can get a tiny benefit from some of the heavy and medium armor passives. The light armor passives don't increase your damage output past wearing five pieces.
Yes, destruction staff is the way to go for a magicka DK.
You should also try to use dual inferno staff for more build diversity ! but as leveling goes don't hesitate to master an other weapon type too as it might come handy.