Ask the Expert – which plug in to use?

Hi,
I would like to set up an ‘Ask the Expert’ feature on my WordPress website. Visitors can send their questions in, and the questions are answered by a team of experts (from their own computer). It needs a set of ‘topics’, with questions and answers below it, and ideally an ability to feature a question.

Any idea what plug-in I should start looking at or which website has done this particularly well?

All ideas welcome,
Inge