Yup, Knock out roses, not the double pedal, go with the standard. They grow like weeds and bloom from spring till December. I throw a banana peel at the base of mine every so often and they are pretty impressive. The leaves also turn a bronze/red color when it get's cold.
Grab a few small pompous grass for accent and your done.

If the realtor thinks it's such a good idea then maybe she would be willing to deduct the cost of the plants and flowers from her percentage of the sale?
