[jerf]

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

Most evidence would suggest that in the Star Wars universe, physics and technology have basically been fully explored for thousands or tens of thousands of years. What's left is style and what exactly is done with it. In the extended canon it is revealed that the Death Star isn't even particularly special and that various superweapons of similar capabilities have been developed off and on in the past several thousand years, determined more by the political environment of the time than the tech environment.

As for the rest, wild handwaving. We don't know how the physics of their space travel works, so it's possible that a moon can not simply be turned into a hyperspace-capable ship any easier than it is to simply build one.

Incidentally, while we're on the topic, here's some relatively-well founded handwaving: http://www.theforce.net/swtc/hyperspace.html