Hi,
For this, I would investigate the following:
Design one fsm that have states exactly like you said:
-- in combat
-- activates turret deploy mode
-- controls switch to the target reticle
-- chooses location for turret
-- deploys turret
-- deactivates turret deploy mode
These states are high level manager for your turret deployment system, in them you will manage player controls, instanciate new gui, and watch for variables or events to know when to switch to the next phase.
Does that make sense?
bye,
Jean