In order to make multi-stage units it is necessary to have 2 or more interim-stage units (e.g. Battleship hull, battleship), each purchased separately and placed separately. If a unit property for number of turns to complete were added, the unit could be purchased and placed but it would not become operational until the correct number of turns had passed. A stage counter displayed on the unit would be nice, too. This has more uses than just capital ships, such as structures requiring more time to complete, possibly 4-5 turns.
If a unit has several stages it is difficult to remember to buy the next stage each turn, but this property would handle that automatically. It would solve some of the TUV issues, too, for units that consume others.