I would recommend a trip to the Lebanon Flea Market on the weekends. The growers should be starting to show up with some plants right about now.

You won't find anything too fancy there, but what they have is very healthy and grows well in this climate. And, it is usually about 1/3 the price of what you'd pay at Home Depot, for bigger, healthier plants.

They have lots of Boxwood, but I think you have to get there pretty early in the morning to beat the landscaping companies. :) They also have things like Firepower Nandina, Knockout Roses, etc. Good ol' tried and true standards, but I imagine you don't want anything exotic or expensive if you're just trying to sell the house.

Good luck!

Having tried and failed to sell right away, I admire you for thinking long term good looks. I recommend dianthus which for me is evergreen and has already started blooming and will continue to fall. I use it as an edger, but it would also look good in the flower boxes. Mine is in full sun and some of it is in the iris bed which doesn't get watered except maybe twice a summer when the irises are really, really dried.

If you are looking for do things rather inexpensively azaleas are about the cheapest shrub you can purchase.
If there isn't anything in the beds already I would do 3 azaleas in a line on each side of the porch. Then add some Monkey Grass/ Variegated Liriope planting 3 in a triangle in front of the azaleas but between the 3 azaleas.

You can also try Craigslist or Freecycle in your area to see if you can get inexpensive or free plants. Sometimes people are wanting to redo their own landscaping and just want rid of some of the existing plants.

A couple of little concrete statuary make it nice too.