The 105mm f/2.5 is the ticket, I would say. Legendary lens and for good reason. If you want something a little faster and can swing a little more (though still not tons on the used market), there's also the 105mm f/1.8. I've owned the f/2.5 version and played with the f/1.8 version. Had I the cash, I'd probably go for the f/1.8. Either one is a solid choice, though.