>>23322120
You FGO experience will serve you well. Patience is the one skill which rules any gacha and you've already developed it. When everyone is crying about no gems you'll be swimming in them.

As for your question, There's no absolute necessities like Castoria or Oberon. It's early in the game's life cycle so we're still in stat-stick mode where very little matters outside of big numbers. That means unfortunatly anything below SRs are all hopelessly outclassed since they have lower numbers and no passive skills. The simplicity does gives you some flexibility since alot of units are mostly interchangeable, so as long as the unit you pick has decent passives or attack effects you can probably use it. The development units are quite good, and those will be where you get the bulk of your roster from. For now just overlevel your Nu Gundam and Quux and leave the story team blank and you'll be able to rip through most of the story stages and get all the free shit so you can start making gundams.