Instead of egg, I'd use egg whites
sugar - part splenda part sugar - sometimes you cannot sub all sugar out. It does contribute somthing to the texture of the dessert
sour cream - I'd use fat free. I personally cannot tell the difference in recipes. When eating it alone I can however. Some people can taste the difference so you'd have to try this yourself. If you aren't sure, maybe use part fat free and part low fat?
Milk - skim milk
Cake mix - allrecipes has cake mix recipes - http://allrecipes.com/Recipe/Cake-Mi...ns/Detail.aspx
you can play around with those to make things "healthier" as well.
Flour - whole wheat flour can make things tougher, denser and heavier and change the texture. If you want to sub part of this with whole wheat, probably would be okay but I wouldn't do all of it
Brown sugar - brown sugar splenda and brown sggar - again - don't sub all the sugar out
Butter - wel some people use prune puree or applesauce successfully. I've never tried that myself.
Leftovers? Send to 3FC people