Lets start with mouse. On mouse click do some line trace that would check do you clicking unit, then you have many ways to move your unit. For example, you can create some actor that always moves with your mouse at screen and when you click at unit attach him to this actor, then detach after release. For this you should use:
This node deprojects screen 2d position to your world 3d coordinates, it would help you to make that actor which would move with your mouse. Remember to make him invisible:)
After release, check does your unit overlap another building and do what you want otherwise, also use your imagination:)
Good luck!
P.S: For touch you can think something similar.