I am in the process of making redesign proposals for a product. The current interface is quite rich on information and options (think of MS word). I want to get an idea on how important a certain option/information actually is. What would be the right way to find out? I have very easy access to people working with a very similar product to do interviews or evaluations.
I think the only thing you should do very soon is to interview users who use the product. The deep understanding of the users who use your product strongly inspires you with new insights and ideas. You must interview customers and all stakeholders.
The things that are very good after the interview are to sort out the insights and ideas you get to improve the product and eventually sort the priorities (the business or anything else that matters to you).
Usage data collected by surveying users regarding complicated interfaces with many options is likely to be unreliable. Users can tell you only what they think they do, but that often doesn't match what they actually do. Consider tooling up the program to collect real-world usage data.