Should I buy it? Help me understand

[primitive]

First of all, planetary weapons is kinda useless. The ship destroying weapons does an excellent job against planets too.

You can not select different targets for different weapons. The ship will move toward its highest priority goal (which cand be planets) and if its in range, fire what it got. If the selected target is not in range, it will fire at whatever else is in range (and the weapon can hit).

If both fleets arrive at the same day both will take part in the action.