If you play Sundisk as your first card does it then allow you to trigger Legion on the next card?

The short answer is no. You cannot play Sun Disc as the first card of your turn and immediately use its ability to ready a subsequent unit.

According to the official rules and FAQs regarding the Legion keyword, Sun Disc does not count itself toward its own Legion requirement.

Why this happens:

1. Legion Dependency: Per Rule 812.1.c, for a Legion ability to be active, a card different than the one with the Legion ability must have been finalized on the same turn.
2. Self-Exclusion: RiftJudge FAQ #9616 and #9805 explicitly clarify that Sun Disc does not count as "another card" for its own Legion requirement.
3. Dependent Ability Inactivity: Because Sun Disc does not satisfy its own requirement, if you play it as the first card of your turn, the Legion ability remains inactive. According to the rules on dependent keywords, an inactive ability cannot be activated.

The Correct Sequence

To successfully use the Legion effect on Sun Disc, you must play a different card first:

1. Play a different card: Play any other card from your hand (this satisfies the Legion condition for the turn).
2. Play Sun Disc: If it is not already in play, you may now play Sun Disc.
3. Activate Sun Disc: You may now exhaust Sun Disc during your Main Phase while the game state is "Open." Because you already played another card this turn, the Legion ability is active.
4. Play your unit: The next unit you play that turn will enter the battlefield ready.