1. Wear what you feel comfortable in. Period. If you feel perfectly comfortable with slacks and a nice blouse wear that. If you are going to feel like a dweeb wearing a t-shirt and jeans and inadvertently act like a dweeb, then by golly don't wear that.
I think of slacks and a nice blouse as dressing down. I think you'll be fine with that.
2. By ALL means, find a tactful way to say "You won't NEED two people if you hire me, so in reality, you'll be SAVING money...."
At salary negotiation time you can by all means lead with that. Let me mull it over and think of a way to put that tactfully....