If a unit with Old Ben's Spirit attached has taken enough damage to be destroyed and OBS is discarded to save the unit does it have any effect on focus tokens that were on that unit prior to receiving enough damage to be killed?

No effect at all.

PBrennan's right. The enhancement specifically says to remove all damage tokens. Any shields or focus tokens stay right where they are.

Edit: You may be confusing OBS with Force Rejuvination which is an event that removes all tokens from the character.

As an extra question what happens to extra enhancements placed on the unit?

they stay put. since this is an interrupt for the unit being destroyed, it never left play, so there is no reason anything should happen to any other enhancements