Hi there,
Yes I agree with you that it is not absolutely necessary for a language teacher to know all subjects to be able to teach their ESP courses (quoting from your letter). However, I still think it will be good credits for the teacher if she/he can prepare themselves with sufficient knowledge of their courses. The students will feel more apreciated by then. With my EMP course, as we are focusing on the academic discussions and presentations, to me then the most difficult part is to choose suitable topics from them. I do sometimes ask them to choose their own topics from their lessons at the university (my students are doctors who are taking their specialists). My tricks are subscribing to reputable English sites like englishforums and medical forums (where I can get ideas for topics). There are two books which are very useful and very inspiring that I would recommend for similar course .i.e. English for Medical Professions (Lorraine Beitler and Barbara McDonald) -- good for reading skills and Professional Interactions (Candace Matthews and Joanne Marino) -- good for discussion and presentation skills.
Unfortunately, I dont know about the PHd for ESP