my buddy over there got it right. Currently the steamdriven airship is unarmed. He has 2 of the max. 3 guard compliments aboard. Just the people he needs to run a ship. I even allowed - since for now they haven't aquired that much reputation - to buy his current reputation debt off with regard to the guards aboard. No need for me to spoil the fun for my group with an airship that goes nowhere.
Most of the ship is driven with steam and electricity, but the cannons should have been arcanotech/magitech (whatever you want to call them). I figured that gouts of flame and lightning a far superior to cannonballs up high in the air. Also you don't want to piss of the lord in the land underneath, just because you defending yourself against adventurous dragons.
In general at least any major power and many of the lesser are prepared to deal with aerial intruders, if those try to be pesky. It just comes with the setting -> crapsack world with steampunk and magitech up to the gills.
His ship has fortifications, but a bristling broadside in a ship-to-ship-fight aren't where I see fortifications, especially if I want to play a raid on his ship out in some detail. His fortifications are that the ship can fly and you need to gain access (challenge 1), you need the resolve to put the elemental/demon creature that is holding the ship afloat under your will (challenge 2) and you need to find the hidden masterkey (challenge 3) to unlock the steering wheel.
The whole buying them with silver thing is what I tend to avoid. You are on the other hand absolutely right that 10 Reputation not that much. I would probably go the way and start with 50 Reputation for a light armament, 80 for a middle armament and probably 100+ for the major league stuff. I tend to be generous with reputation for several reasons.
I like your idea of carpet-bombing and launchable dragons.
Nice thing with the nets in your scenario... clever and deadly...
just aquire more influence in the high houses of magic and he can even buy a band of arcane warriors as a mercenary crew ready to kick even dragon ass.