I believe it's Monday the 25th, but you should check sources. Or until the 5000 hulls run out. Currently around 4400, you can see the exact number when you look under Other Purchases tab on the Buy Phoenix page.
I wouldn't call it a distinct upgrade. It can have a max size 6 power plant, which is higher than the other Connies, and it will have a dedicated point defense battery. That said, it's still very much a luxury vessel. You can upgrade its components to be a yacht with nice armaments, but it'll never have the customization possibilities of an Andromeda. I personally believe all of that customization trumps the size 6 power plant, but it can only be pure speculation at this point. We won't know what a size 6 versus a size 5 will allow until we're playing the game.
With regards to the Aquila, I'd generally advise (if you buy the upgrade) not applying it until we know a lot more about the ships and are closer to playing the game. You can always melt an unattached upgrade if they develop in a direction you don't like.
I'm leaning towards the Aquila myself, but I want to know more about how any of these ships customize, and so will be buying an Andro and the Aquila upgrade separately and waiting for more info from CIG. It may be that I can get what I want from the Aquila in a customized Andromeda (minus the sweet command deck), though at an expected steep cost in-game.