Hi wahrez:
All three lenses you listed are capable of capturing excellent portrait photos. The 50mm f1.8 is also a steal in terms of cost for such an outstanding lens. The 85mm f1.8 is Nikon's "bargain-priced" 85mm. The better portrait lens is the outstanding, but pricey f1.4. It is legendary not only for its sharpness, but also because of its prized bokeh, i.e., the visual quality of out of focus highlights and blur.
For a D100 with its 1.5x FOV, I have found that full body portraits or small groups of two to three people benefit from a wider lens than even the 50mm. My favorite for one or two people is the Nikkor 35mm f2 AF-D, and for three people, you may have to go to the Nikkor 24mm f2.8 AF-D. As you can tell, the 24-85mm zoom would cover all of these ranges, but perhaps with slightly less sharpness. Further, you probably will not attain the sharpness of the primes while using the zoom at the wide open or nearly wide open f-stops that are favored for portrait photos. Thus, there are trade-offs to consider. If price is no object, or if it can be rationalized, by all means go with the primes for portraits, and substitute in the 85mm f1.4 if you can possibly do so. It is worth it.
Also, if you can live with manual focus, don't forget to consider the 45mm f2.8 P, another legendary lens that is also useful for architectural shots.
There are lots of lens possibilities for you, I hope you have fun making your choices.
I'm thinking of buying a D100 and have seen the following lenses
recommended on here:
Nikon 50 mm f/1.8 D AF
Nikon 85 mm f/1.8 D AF
Nikon 24-85 mm f/3.5-4.5 AFS G
What I want to know is what each of the lenses is ideal for and do
I need all three? I'm looking to photograph portraits (Head and
Head/Shoulders) and also full portraits of people in scene.
Thanks,
Wahrez
--
Bill Adams
http://www.pbase.com/bill_adams 
