[Alneyan]

Note that a Star Destroyer is very cheap by the end game: it only costs twice as more as your regular baseship, and three times (or so) more than a Dreadnought, if memory serves. The actual cost will vary depending on what you put on your warships, of course, but my point remains the same: Star Destroyers are not really expensive in the later stages of the game.

For my purposes, the Nebula Creator component is used on Stellar Destroyers. If you are willing to pay extra and want to make sure the targetted systems are quite definitively dead, the Black Hole Creator is the way to go instead. Either way, nothing should prevent you from blowing up your own systems, should an enemy fleet arrive: that will save you the happiness hit, while destroying the enemy fleet, and slowing down further attacks. Just evacuate the system beforehand if you care about those suckers... your most esteemed citizens.