1) No. The people getting the Mantis Armor are shoplifting using the pre-"nerfing" patch. Personally, I have no problem with it because a) it makes things more realistic, b) it's more of a challenge and c) it's the way the game was intended to be in the first place, according the Curries.2) Yes, after 24 hours he will have more stuff to PP but it's unlikely you'll get enough mantis armor this way for two. It's easier to buy it legitimately and with the Alchemy merging skill, it's quite easy to build up a lot of cash in a short time.
3) His custom items are Mirror Armor, Vampire Chain and Ivory Blade. He also sells things like the Infinity Helm, Snakeskin Boots, and the Cloak of Many Colors. My advice is to have at least 500K before visiting Ferro because you'll need it. His prices are steep and he has lots of good stuff. To the best of my knowledge, no one has published a list of everything he sells.