First question is which fuel will she be using- gas,electric,oil, heat pump?
Next is what one of the other posters said- They are all pretty much the
same. I prefer Trane or Ruud/Rheem but only because of one reason---Make
sure you can get local repair work done by a reputable service organization!
Ask some neighbors who they use don't be afraid to call them and conduct an
interview. It's a lot of money at stake (both for the furnace and the fuel)
and you want to make the right decision.