Have to agree with Erik on BW. The subject of this image, from what you've said, is the removed sign and it's story. Converting to BW will reduce distraction from the blues/greens and focus attention on where you want it. Also helps in giving a "looking back" feel.
On adding a nostalgic feel to an image, you really have to take the image into consideration. This might sound nit-picky, but casting nostalgia on this particular image doesn't work well due to the fact that the sign has been taken down. It doesn't invite you to reminisce about when the owner and his horse farm lived, but confronts you with the present in which neither do. Which is why BW is a good option, as it draws attention to the removed sign. Were the sign still up, it would be a different story.
Always decide what the focus of the image is, and choose your PP towards that end.