Help me purchase a vehicle

I have finally hit 1k of necrons! However, nearly all of it is infantry. 20 warriors, 10 immortals, 5 leaders, some skorpekhs and flayed ones and deathmarks and wraiths. The only anti-tank units i have are 2 heavy destroyers and a doomstalker (which are admittedly quite strong). However, I feel like my dudes are getting shredded to quickly and I only have 3 dudes to punch back at their tanks. Although in reality I just want an excuse to buy more models.

Anyways. Necrons have many vehicles. The barges, triarch stalker, doomsday ark, obelisk/vault and monolith. Which one is right for me?

Edit: Okay if not a DDA then what's the next best thing