For an escort bridge I suggest the Belfast Bundle. It is expensive, but the other bridge packs are not worth it to spend any C-points on them.
For a Starcruiser use the Bridge of the Odyssey if you managed to pick one up on the anniversary.
250 Members for a fleetbase? No thanks. Then ZrekBBS will never have one.
But upgrades and extensions for bigger fleets are acceptable and a nice perk for the bigger fleets. Also isn't there a hatd cap if around 300 toons in fleets anyway atm?
So a requirement like that would pbly benefit a real small number of them.
There should be purchase restrictions that require a few active members to raise the necessary resources. Make it so expensive that a couple players will never be able to buy it (so EC is out of the question).
One idea would be successful Fleetaction and STF private runs that award the fleet X numbers of Starbase tokens to buy a standard base and later upgrades.