Shameless plug to my build thread (gameplay and build videos included!).

Specifically regarding your sustain problem, you absolutely need percentage based cost reduction on a MagDK if you plan on solo or small scale PvP. Your options are;
1) Light armour
2) Seducers
3) Alteration Mastery

MagDK's skills are prohibitively expensive and got hit hardest by cost reduction being removed from CP system. If you are wearing heavy armour, Seducers gives you more sustain then even Lich (assuming pretty liberal use of expensive skills like Talons, Coagulating, Inhale, Wings). Personally I would recommend light armour, even in a 5-1-1 setup you are still getting a huge 10% cost reduction, combine that with Atronach mundus and careful usage of pots and ultimates and that is as much sustain as you need.

Alteration Mastery is another good set that pairs well with DK's Battle Roar passive, it reduces all your ability costs and allows for more frequent ultimate usage boosting sustain even more.

Hope that helps!

There's a couple ways to go about it.
Since launch I've preferred the s&b option and getting my sustain through sets like desert rose, syrabane, blackrose or bloodthorn. They've all had changes and a more or less effective today.

Something like 5 desert rose + 5 impregnable armor + undaunted set (5 light armor) still works well today in outnumbered situations.
You can use bloodspawn to increase your survivability and sustain through faster ultimate gain, or Thurvokun if you want to trap enemy players in small corridors and debuff them using banner/ chocking talons and being a great help to your group that way. Skoria for some burst damage and extra hp.

I also like to play Destro/resto with something similar to your setup, very high damage with relatively low sustain so that fights end very quickly before you run dry. The resto staff gives you access to healing ward which can give you a bit of extra time to heal up with dragon blood, heavy resto attacks to increase sustain and more ulti regen if you consider that range attacks allow you to start regaining ulti faster than if you only have melee weapons.
In this setup you can try the golden food which gives you max health, max magicka as well as health and magicka regen. That will give you something around 1350 magicka regen unbuffed, which will go higher with the continuous attack buff and potion usage.

with a magicka DK, you MUST use a lot of heavy attacks, personally I dont use S&B for this very reason...I am willing to give up the extra defense to be able to restore magicka on either bar(would rather have the extra healing and extra resource return from heavy attacks than the extra defense from S&B). Its true I am less tanky...but very bursty, its a trade off for your play style.

regen on some jewelry will help, as well as some resto heavy attacks. vamp could also be helpful like you said