With regards to how long your missle launcher will last. An attacking fleet can only wipe out 30% of your defence in 1 wave. Not sure how the game figures that out if you only have 1 of something and lost 30% of it! lol. The other 70% of your defences are considered 'Repaired'.

Here's a good tip too.

I launched my fleet earlier, a whole 2 light fighters and 2 small cargo ships to a planet that had 1 small fligher and 1 missile launcher. Running a speedsim I was 98% sure to win. 5 mins before the fleet arrived at the guys planet I sent out another probe to see what was going on. The guy had built 2 small lasers while my fleet were on route. So I recalled them in the fleet screen. His 2 small lasers, 1 light fighter and 1 missle launcher would have taken me for sure.

If you can, always send a probe a few minutes before your fleet arrive, just to make sure nothing has changed in the hour or 2 that your fleet are traveling to the destination.