I think the 105mm is a little short for a portrait lens, especially if you will be doing head and shoulder shots. The classic lens for 6x6, Hasselblad, etc., is 150mm. For my Bronica, I have a 150mm but the 180mm is used alot with the 200mm used by some. I think the 105mm is closer to a 65mm equiv. in 35mm, a little short. In 35mm, the portrait lens range usually is 85mm - 135mm. I'd look for something from 135mm to 180mm. The DOF of 135mm @ f4.5 will be close to the DOF of 105mm @ f3.5. The 105mm might be fine for waist up portraits.

Brian