Hi!
1) I think the medium would be fine on you. I'm 5'3 and 110 and I have a large lol!! I got my large on clearance and am actually considering a medium too. If you want I can post a mod pic with my large.
2) I definitely think you could carry blush in the colder months if you wanted to. I have a pearl grey Selma which is also a very light color and I've carried it all year. As long as it goes with your fall/winter wardrobe I think it would be great!
3) I would say that's a good price. If you waited you might be able to find it cheaper but it could also sell out. If you don't mind me asking where did you find it at that price? I've only seen it at
Nordstrom