[freehunter]

>If a Death Star takes a couple of decades of build wouldn't be obsolete by the time its finished?

Only if there's something better in the meantime. Which there really wasn't in Star Wars. Also it can be built while it's in operation, like a supermarket remaining open during remodeling.

>a planet sized space craft from metal seems silly when there are plenty of large space objects ready to be commandeered and turned into very large space vessels.

Would a planet be able to retain its structural integrity while being carved out to house a million soldiers? Having to build structures on the surface would defeat the purpose of a Death Star.

Also the Death Star has defenses, so it doesn't traditionally need to withstand large impacts.