Ships with no attack no defense

IIRC, a planet cannot be invaded if the player has a ship in orbit.

Any ship, with or w/o armament.

...and they can be upgraded later.

True, but easily defeated by adding one basic weapon to the troop ship design. It takes an extra turn to invade, but it can work in those peripheral areas where no one has real warships around. Always seemed to me that the AIs were overconfident about those 'neutered' defenders.

Note that I did not praise the AI tactic, just that there is a basis for the script that directs it.

In addition, forcing the expense of the addition of weapons onto invasion transports (that go away), has some value, as well.

I would also note that (at least in DA) some AIs put weapons on transports, perhaps anticipating " 'neutered' " defenders.

Or a military startbase being placed close to the planet can give some attack values to these ships with appropriate modules on it, but I don't remember seeing any.

Speaking of said defenceless ships, I build them early in the game so when I do get weapons tech, I can just upgrade them.

Poof: instant military!

I don't think it would be cheaper, but is it faster?

Building hulls and doing an upgrade is much faster than building real ships, yes.  Provided your ships are within range of a friendly base/planet, they'll only take until the next week (that is, turn) to upgrade-as close to instant as GC2 gets.

Building hulls and upgrading them is usually cheaper than buying outright-but it's still quite expensive compared to building it with production.

Since the function for base rush buy cost is trunc(cost^1.1)*6, this means you're paying anywhere from about 8x on the low end to as much as nearly 14x by the time you get to a ~4000BC production cost ship (which has a no-lease rush buy cost of 55002BC).

Upgrades seem to be a little more complicated, particularly when involving troop modules, but the basic premise remains true.

I thought that a ship had to have at least one weapon or defense module to get bonuses from a military starbase? (It's been a while since I played very much.)