The "Red-Light-District" is nothing to worry about. It is more or less in the quarter Niddastraße, Moselstraße, Taunustraße, Elbestraße. It is safe to walk to and from the hotel from the main station. And if you look at Google Maps there are a number of hotels in this area. I stay there frequently, but split my work stays between this and the close by Hilton Garden Inn if I need to be in downtown Frankfurt. The rooms are nice, well maintained and the staff is friendly.