By 'defense' do they mean shields and armor, or the capability to fight off attackers...?

I too don't like the idea of mobile star ports.

Choose the tile to build it in, within X tiles from the constructing planet.

Don't give it weapons by default, just give the option of building basic defensive weapons with a maintenance cost if built.

If you need to put in a better location, decommission it and build another.

Long term protection will require a docked defence fleet.

I actually like this idea. I simple value that can be modded up, down or out is a great idea. Put it a low/medium value at launch of game and allow fellow players to mod or adjust it as they desire. Again putting the taste of gameplay back on the player instead of hardcoded choices that others may not like on down the line. 

Yes lucky it does make for a cheesy road block and lacks any credibility, however it would solve the problem and keep the solution simple both at the same time. 

Give Stationary objects & Star ports a 'combat' rating which is really just an indication of how strong the object is (it includes, gun platforms, AA, shields built deep in the platform, ect.) Fleets have a numerical value we look at anyway, you can simply say your rating must equal or exceed 40 to attack and destroy this object, 3 drengin bombers each with '6' missiles could not touch it, but 6 of them would or a ship that had both defense & shields added up to equal 40 would. 

Simply tossing ideas into the creative Alpha Pot so to speak. 

What's cheap about it?  Just say that the amount is the damage the base can repair easily, so if a fleet can't exceed that, they can't destroy the starbase.

I think a 1hp litter box with a peashooter shouldn't be able to destroy a hulking starport easily.