If you buy this house, I'd come early in the day to scrub out the
bathroom (including the walls). Move your cat's litterbox and etc.
into there, and make that her temporary home. Plug in a Feliway
diffuser to help calm her, and you can put a few drops of Rescue
Remedy in her water too. Bring a towel or blanket with your smell on
it for her bed. When you're moving in, keep the door closed! You might
even put a sign on the door saying "cat inside, do not disturb." so
she doesn't run out.

If you really feel she will have a hard time with other smells, you
can steam clean the carpets and wash (or paint?) all of the walls
ahead of time. Plug in Feliway diffusers around the house. S-l-o-w-l-y
let her have access to other parts of the house, as she feels
comfortable.