I think the server should skip the 1NPC-All Item Seller idea and simply create a custom gachapon system.
Every server i've ever played with a AllItemNPC has a few glitches in it. They also allow for mass scrolling and items are made too easily. Unless the prices turn out to be astronomical, which they rarely are, the NPC isn't the best option.
If each gacha system was custom with Class based items (i.e. Kerning has Stars) the server would be a little slower to fill up with godly items and rebirthers after rebirthers, and the server'd be a little more fun
After the gachapon is done, just make all monsters drop gacha tickets at a low rate and voila