Last year I looked into getting a Fujinon 180mm to add to my 135mm for my usual 4x5 set-up. After looking at several auctions, and KEH, I gave MPEX a call. Seemed they had one in stock. Then another call confirmed that the Fujinon they had was not what I might want, though they offered me a Nikkor-W 180mm f5.6. I took them up on the offer, and after trying out this lens on a few shots, I was very happy at the recommendation. Basically, if you can find a good example, I think you would be very pleased using one.
One note, if by portrait you mean tight head shot, then a 180mm puts you very close to your subject. Even a 210mm would be quite close. If you only want to do head shots, then a 300mm might make more sense, and give you a more comfortable working distance. On the other hand, if you are doing more open portraits, or closer to full body, then a 180mm provides a nice amount of coverage and field of view at a comfortable working distance. You might also want to add the small Nikkor-M 200mm to your list, though they are not easy to find at good prices.
Ciao!
Gordon Moat Photography