I think the spray foam in itself is a great idea and would be technically superior to anything else. Spraying foam will fill voids and cracks that can't be done with batts or rigid foam. Doing spray foam isn't cheap and the overspray sticks to anything and everything that isn't covered and sealed tightly and is difficult to remove afterwards. Definitely do the ceiling, as that's where much of the heat loss is and you'll end up with a much better R value compared to batts. Also do underneath too if feasible, but rigid foam would be okay there.
If you do ahead with this, please post pics!
