I would:
- review the NDA, and make sure your lawyers are happy with it
- disclose as much information you feel comfortable with
- if they want more, ask for a term sheet (with a number in it, and all details about a possible earn-out etc.!)
There is no pressure on your side as you are not looking for an exit yet, but I assume there is a price for everything so just keep moving the proces.
PS If you haven't yet, read all M&A articles here: http://mba-mondays.pandamian.com/tableofcontents/