Thanx! 
It's not just time, but ship count and location. That is, it might take 45 turns or more to build one, but just a few to build a stripped hull, then one or two more to upgrade. So, I could get many in the time it would take to build one. Rush buys are an alternative, with letting the builders work a few turns to reduce cost a little, maybe. However, using the stripped then upgrade method allows stockpiling of bare hulls to be upgraded on demand. For example, one can wait to upgrade until the desired weapon is researched. For another, one could have stripped hulls in orbit around many planets to avert troopship-only invasions, and then upgrade them if a warship or fleet or escorted transport approaches. This last one avoids the expense of expensive ships around worlds that never get threatened.
Of course, the answer to the second Q is great to know.
I know the bonus is supposed to apply only to attack, but I sure thought that I saw some increase in defense. I have a saved game somewhere and will build identical ships, one at a bonus planet and another at a non-bonus planet and try to confirm that I am mistaken.