I understand your question, for d4.. you should not need anything more that a "competent" doctor, I would lean more to a message doctor as well, knock on wood I have at this point had only 1 one week injury.
I have had lesser rated doctors and had no problems....but....you only need to get burned once to understand what I'm saying, I know you do.
You are in d4 right now, right there where you are is where you invest in your infrastructure, right now, not when you promote. With a good sized arena, you can invest in a good staff and make a profit, provided you dont overspend on players.