I just completed an interview a couple weeks ago - consisted of one panel of 5 people. Reps included folks from HR, Ops and Training. I wore dress pants, white shirt and tie. They commented that you can always spot an interviewee by the tie, so apparently its pretty standard dress. Don't think wearing a suit or sportcoat would hurt, but probably not necessary.
As far as demonstrating ambition for getting into license class, I'd tread lightly. There are many people on this board in position to give a more informed opinion than me, but since I was recently sitting in the interview chair, I'll throw in my
. I went in with the mentality I'd try to be as good an NLO as possible, and when my performance indicates I'm ready for license class I'll be happy to go - that's what I tried to convey to the interview panel.