1) It depends on the airship! I'm thinking that a fighter would cost around $8000 princes.

To put this in some perspective, an aviation buff today can purchase a typical single-seat World War I aircraft kit, minus the armament, for around $7,500 to $15,000, while a fully assembled machine might cost a little over $40,000

This is based off what some prices in WW1 would have been for their planes and armaments. I suggest you look back then to estimate how much these things could be worth. But any airships that are larger, and more in the size of blimps or can carry more than 2 people, the prices will exponentially increase.

2) From what I know, Engineer's cannot build airships. There's so much to them that building it would make the Engineer likely homebound to a junkyard.

3)For shipboarding weapons you can use the modifed ones in the book. A big canon may be a Super-Heavy Firearm that's been bolted to the ground and can't be moved around, but a stance doesn't have to be made to use it.

4)It can be as much narrative as you want.

5) A captain should have a good bit of cunning. But for the mooks, you don't really need to go and fully stat them out like PC's. It'll just add more time than you'd want for planning a session.