in my opinion, whatever of the classes or specs can push out the most AoE damage as quickly as possible will be the best choice.
I'm also looking into, for me, a second twink. I had thought of elemental shaman,but having played it at 110, there is a bit of a build up associated with the spec to gain enough power to release the AoE.
Was also considering warlock, but don't know enough about them yet to decide. Like you say, it would be to invest a lt of time and oney in it.
When i played DK in legion, i'm not so sure if it's suitable, blood might be better for "on tap" AoE.
I have really cheap gear on 1 realm for an ele shaman, so it's not much to invest, but to consider lock would need some more research. Keep me posted.
For the gear type, it depends on how much a class will benefit from the bonus stats. As a Vengeance DH, for example, i don't benefit much from 700 mastery compared to 400 or so agility, so i'd look to drop my gloves asap (hopefully tomorrow HFC) and also gain the 5% bonus to main stats from being completely leather.
In your case , Unholy would benefit more, so look towards leather, mail or cloth with Mastery on the slots that are not BoE plate. Maybe from mail, you still get strenght?