Story Mode Requirements: Level 50...
Hard Mode: If you know what you're doing, level 49 epic mods (the ones you get from dailies) are good enough.
Nightmare: Full Columi is recommended.
Don't listen to this. You'll have a really, really tough time unless all of your players are experienced mmo players.
Normal: Full Daily 51/50/50
Hard: Full Columi
Nightmare: Rakata/Columi mix