Ronny, I work in pharma and frequently have to do similar market research.
First of all, and I can't stress this enough, you MUST maintain a tone of neutrality. It's incredibly easy to bias the respondent and a tone or gesture can influence the response.
One of the most important, and most challenging, is to order questions in such a way that the sequence leads the respondents down the path of thinking you have chosen.
I concur with a lot of riskophile's points. He/she has laid out some effective questions. I will provide a very basic "bone" structure:
Current standard of care: Try to flesh this out as MUCH as possible. Often times, explicitly asking physicians a question causes them to respond without properly thinking about what you are asking and the implications of what you are asking may/may not have. I hope I have explained this clearly. By establishing a very well-thought out standard of care, you can start probing points the nurse/physician has made without being explicit and get a "feel" for their response. Then when the question is asked explicitly, and their response is contradictory to what could be assumed earlier from your implicit question, you have a clear channel to help the nurse/physician think through it and provide you with very well-reasoned answers.
Outline competing products/therapies: In your interviews, ask questions about pipeline products/therapies that are your competitors. This will help you further structure not only the nurse/physician's thoughts but also your own as you direct them to more explicit questions about your product. Their responses for this question, will give you great thinking material for things that are necessary/unnecessary for your own product.
Your product: Ask pointed questions about what they consider necessary for your product to be successful. What are critical features about your product? What are "nice to haves"? etc. Although these are questions you have set, keep in mind that they will evolve as your interview goes on based upon earlier questions.
In terms of presenting to investors, these are qualitative filters to assess the value of your project. It would be good to follow up with a survey to quantify these responses as a concise way of presenting to investors.
Good luck! Feel free to contact me if you have any questions!