Light armor is better than heavy. Magicka DKs have a lot of great damage self-healing, and light armor will really boost your magicka sustainability. (As I used to always say about my Dragonknight, "If I run out of magicka, I am dead.)

My Dragonknight healer can not only heal effectively but has also heal-tanked when grouped without a tank, including some rather interesting runs of Veteran White Gold Tower. If you are worried about survivability you can do heavy chest, medium legs, and the rest light.

I agree with the suggestion of Julianos. It's an awesome crafted set, and even good for most endgame content (excluding veteran Trials). I am not such a good resource for seconds sets, unless you want to talk about playing a healer instead of a damage dealer.

The Infallible Aether jewelry is currently considered the best, although that requires running Craglorn Trials. Willpower jewelry is still in solid second place. You can re-enchant the jewelry with Spell Power enchants ("Spell Harm"). If you are still leveling, I generally do not bother with set jewelry, but just pick up things with the Arcane trait (or Healthy if you are feeling too squishy).

If you are endgame, the current favorite monster helm seems to be the one from Crypt of Hearts I. If you are still leveling, of course, don't worry about this right now! No sense worrying about getting endgame gear until you are lvl 50 with at least 160 Champion Points.