Personally, I'd be looking to keep the rats out permanently. Yes, you could probably come up with some solution to just keep your girls from getting pregnant, (i.e. housing them in a slightly smaller cage inside a larger one) but again, these wild rats might be carrying diseases that your girls aren't conditioned to, and you could lose them. Beyond that, if the rats can get in, your rats can get out... they'll find those exits, and explore them quickly enough.
My personal thought would be to pick up a few square feet of thick (thick enough so as to not be chewed through,) chicken wire, some wire cutters, and a staple gun. If you're moving in, the place should be pretty empty when you arrive, so very thoroughly inspecting the whole place, including "hidden" places (i.e. under the fridge/stove, and the like), and cutting and stapling the chicken wire over every hole, no matter how small. Open up any vents (i.e. dryer vent, air intake etc.) to the outside, and do the same thing: air will still flow through the chicken wire, but the rats will not. Rats are not magic, and their gaining entrance means there is, simply, an entrance for them.
Personally I'd see this as a positive thing for living arrangements period: keeping out wild animals can only be a good thing for your home.