They aren't ultra rare. The number of 20 is absurd. I own one, and have seen several others for sale at gunshows, so the number is certainly higher than 20.
The CZ-83 in 9x18 is basically a commercial version of the CZ-82. The 82 was the standard Czech pistol from 1982 onward. The 83 chambered in 9x18 is the same, except for the marking. You can also find CZ-82's here in the states for a decent price, so they aren't as rare as hen's teeth either.
They are fine handguns, well built, reliable, accurate, and everything you'd expect from a CZ. The interesting thing is the 82/83(9x18) has a barrel with polygonal rifling instead of cut rifling.
Mine didn't like Wolf, but S&B and Hornady JHP were quite accurate. It's been 100% reliable. The S&B and Hornady hit in the same place, so at least I can use cheap practice ammo. It has an Ambi safety and is SA/DA and cocked and locked capable.
They're a tad on the big side for a pocket gun, and are a bit wider than a Makarov because it's a 12 round, staggered mag. The 13 rd .380 mag and the 12 round 9x18 mag are the same mag. The 9x18 is a hair wider, and it just won't fit that last 13th round. Same mag though, so spares are easy to get.
CZ-USA may have quit brining the 9x18s into the US, or maybe CZ quit making them, but there's plenty of them here that it shouldn't be much of a price difference from one to the other caliber right now. I'd pay a couple extra bucks for the 9x18 because I think it's neat, but it's not so rare that you should pay an arm and a leg.
You did indeed find a gem, and it's worth buying if it's a decent price/shape. They're very nice guns.