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.
I have a 85/2 and two 105/2.5's (one with inbuilt hood, and one without)
the 85/2 gets a bad rap sometimes but it seems sharp enough for me (in the same league as the 105) when stopped down a little (f4 - f5.6)
If you're working inside, the amount of room you have versus how much of the person you want to capture (headshot, upper boddy) and how close you are comfortable working all weigh into which one to pick. IMO of course!
Nikon has many excellent lens for this kind of subject, the 85, 105, 135 and
180mm focal lengths all are superb but the 105mm f/2.5 has the best size,
wheight and performance relationship followed by the 85mm f/2, or the
135mm f/3.5 in my opinion. I love, also, the old 85mm f/1.8, the 135mm f/2.8 and
the 180mm f/2.5 but they are bulkier and not so "portable" like the first ones.
Micro-Nikkor 105mm f/2.8 AIS has my vote. Tack sharp and dual purpose. I use a yellow-green filter for hiding blemishes. If you want to do color portraiture, you may want to use a soft filter (I never have and do not know anything about them).
Not to know what happened before one was born is always to be a child - Cicero