Caitlin, Mai-Lynn, Lovisa, Rebecca, Amelia, Averil, Valerie, are the ones that come off the top of my head. Rosita and Eliana have their quests tied to the acquisition, same with Leia, but they're also purchasable.Are there any unique NPCs among those you can buy (aside from Aria and Stacey which are quest-led)?
Edit: Just realized I might have misread your question. If you're asking if you can buy unique characters, yes. Everyone from the Kyamanto syndicate is unique, everyone in the gallery at Crystal Heights is, and there's a chance to get randomly appearing uniques in the auctions, like the Dixon girl and the Stormchild.