Roxy and Paul - I obviously don't have much experience with different types and qualities of paint. Once I'm experienced enough that I think I'll be able to tell the difference between artist and student grade paint I'll have to try using purely artist grade. But for some colours, like Phthalo Blue, which you only need a small amount of to mix in with other colours, I wonder if it will ever be worth it to spend $50 on a tube of artist grade compared to a $5 tube of student grade.
When the paint seemed clumpy I just added more SDM and kept stirring and it smoothed out.
A question for you - does burnt umber always smell so rancid? You'd think it would just smell like cloves from all of the clove oil but what I'm experiencing is something different
Let us know whether you find the photo as simple to paint as you're anticipating - I'm interested to know.
As for brush size it probably depends on the look you're going for. You could do it all with tiny brushes, or you could do it all with medium sized brushes like Mark uses (he says he even uses medium sized ones for putting the tiny little reflection dots in people's eyes).
If you're doing this as a learning experience it might be fun to simply avoid getting bogged down on tiny details with tiny brushes and instead just try and follow the Carder method straight-up and see how it turns out. Of course you can also just save the tiny brushes for the eyes and other detailed parts.