I suggest you read these posts on study of Materia Medica before you decide what Materia Medica to buy:
Reflection on Materia Medica Study
Links to Free Materia Medica Books
James Tyler Kent, Lectures of Homeopathic Materia Medica
Constantine Hering, Condensed Materia Medica
Constantine Hering, The Guiding Symptoms of Our Materia Medica
Link to Review of Hering's and Kent's Materia Medica