If I play Void Rush can I exhaust Sun Disk before playing my unit from Void Rush?

No, you cannot exhaust Sun Disc in the middle of resolving Void Rush to affect the unit played from it.

Reasoning

1. Sun Disc lacks the [Reaction] keyword: According to Rule 369 (Activated Abilities) and the established rulings for Sun Disc (see FAQ #5683), Sun Disc can only be activated at base speed when the game state is "Open" (no chain is present and nothing is currently resolving).
2. Void Rush creates a closed state: When you play the spell Void Rush, it is placed on the chain. While Void Rush is resolving, the game state is "Closed." You cannot activate Sun Disc during this time.
3. Legion Requirement: Sun Disc's ability requires the [Legion] keyword to be active, which means you must have already played another card earlier in the turn. Even if you could activate it, you must do so before you play the spell that would put the unit into play.

Correct Sequence

To have a unit played from Void Rush enter ready using Sun Disc, you must set up the effect beforehand:

1. Play a card earlier in the turn to satisfy the [Legion] requirement.
2. Exhaust Sun Disc while the game state is Open (no chain present).
3. Play Void Rush.
4. The next unit you play (in this case, the one from Void Rush) will enter ready.

If you play Void Rush without having already activated Sun Disc while Legion was active, you cannot "interrupt" the resolution of Void Rush to activate Sun Disc.