I love seeing new 8Rack players!
Some cards that are worth the investment are
Inquisition of Kozilek,
Thoughtseize, and
Urborg, Tomb of Yawgmoth. A little pricey but they can help a ton with single-target discard and making your black-mana-intensive mana base work well with your man lands.
Liliana of the Veil is not a cheap card, but if you don't mind the extra mana
Liliana, Waker of the Dead works just as good if not better than her 3-mana cousin and even has a sick ult that by itself can win you the game.
For extra discard I also like
Wrench Mind in a not-artifact-heavy meta and
Raven's Crime makes use of your extra lands if you've already topped out on your curve.
People talk about
Urza's Saga for 8Rack and in my experience it's not great but if you can afford a few you might have a better experience than I did when I was testing.
Rootin' for ya! Rock the [heck] on!