Ideas and suggestions.

The power of magic probably doesn't matter here, as sending an eagle to the enemy is more of a skill than a spell. The range can be set equal to the scouting range of that hero. As for requirements and limitations, I think a single obvious requirement would be sufficient – a certain level of wisdom. As for "for 1 hero," it's obvious that the spell only goes into the spellbook of the hero who uses the "keen eye" skill.

P.S. another small addition to the "eagle eye" – all spells used in combat against opponents and neutrals are instantly added to the hero's spellbook, and they can be used immediately in that same battle.