Hello Will,
Good luck on the portrait. I'd like to see it when it is finished. There are two things that come to mind when looking at it. I believe there can be potential pit falls, so I'd like to mention them. First, your photograph was taken indoors, with artificial lighting. The light was close to the subjects. An outdoor scene that you have sketched will require different lighting. Can you shoot a group of photos at the scene that you'll be painting? Will the commissioners let you paint an indoor scene that will work better with the photo you've chosen?
The other thought I had was one of human emotion in a painting. It is a personal thing with me, so forgive me if it's of no interest to you. I hope it can help though. Smiles must be done very well. If not they can look superficial. Leonardo Da Vinci, Rembrandt and Ingres are artists that can teach worlds about creating emotion in a painting without going cliche or superficial. On thing common in each is that the models rarely smile. What is more common is a slightly up turned mouth that shows a positive state without pushing the "happiness", for lack of a better word in the portrait. Well, good luck with it.
