You can buy the Luxury Yacht on the two OS&C planets; Curacao and Baden-Baden. Isn't a great stretch to guess that a ship devoted to luxury would be sold on planets devoted to luxury.
As for the unpiratable thing - yeah, that's sort of cool, but I'm willing to be not a single person bought the yacht to trade and get pirated in.
Personally, when I read about the Luxury Yacht, I envisaged essentially a gunboat. Compared to other gunboats, it would be more agile (or similar to a gunship), bigger, and have less hull, powerplant, and turrets (or it would switch some of the GB turrets for transport turrets).
As long as the Yacht stays on 1500 cargo or below, and can't kill another gunboat (unless you have set the yacht up exclusively to kill gunboats, and it runs in to a gunship set up to kill light fighters or whatever), I don't think people will say it is too strong.
These are my impressions, anyway. I'm still going to buy the luxury yacht, because RP-wise it really fits what I want, almost exactly, and many times closer than the next-nearest options, but yeah, in terms of everything except RP, it seems like it is pretty terrible, and almost criminally expensive.
