Actually, you wouldn't necessarily need the building either. The complexity of the script would all depend on exactly when and where you wanted to place the unit. The simplest one would be the creation of the unit in a specific city in a specific date for the player. The most complicated would involve a variety of different triggers, citing the unit in different places and giving it to the AI (which you'd be best off having a background script for, unless the player's willing to go along with it).