I'm mildly allergic, but I just ignore it. I also suck at keeping my house clean, that would honestly probably take care of it.

As for stuff you need to have, really just a shit box and some food. Whenever I'm introducing a cat to a new house, I always lock it in it's shit room (laundry room, generally) for the better part of a day or so before I let it out to explore the rest of the house. Let them establish a home base before they get all freaked out.