In general, if you see a card that is selling on TCG Player for below Card Kingdoms bulist price and the graph looks like this, thats probably a safe place to park some moolah
There's almost no downside, and putting a card like Ulamog, the Ceaseless Hunger or Karn Liberated underneath Ugin's Labyrinth via imprint only to pick it back up when casting it is easier adds a layer of versatility to the Tron strategy that will make it even more of a threat than it has been previously
Thalia, Guardian of Thraben increases the cost of non-creature spells, Kira, Great Glass-Spinner protects your creatures, and Shalai, Voice of Plenty gives you, your creatures, and your planeswalkers hexproof
Since you're the Rakdos player, you benefit when your opponents suffer damage, so this minimum ping of one damage per turn means a lot