Yes. Make sure you purchase your sigma lens with the F-bayonet. Here is their lens finder: see related link
Yes, the sigma is a good lens for the money. One cannot expect professional result with this chepo lens but for an enthusiast it will serve the purpose.
Sigma has a 70-300 zoom lens that is comparible with the Nikon F55. It costs around $150.
No http://www.amazon.com/Sigma-100-300-F4-5-6-7-Nikon-Manual/dp/B002CJ03TU
and nikon camera can use a nikon lens, try a D3000, a D50, or a D9
Nikon or Nikor
There are many good Nikon lens for portrait photography. However, perhaps the best Nikon lens for this purpose is the Nikkor AF-S 50mm f/1.4G lens. The cost of this lens is approximately $450.00.
Af-s and Af-I lenses are compatible. Listing them would be hard, considering tamron, sigma and nikkor lens can fit on a d60.
The Nikon d2h camera has the ability to adapt to many different lenses. The lens that comes from the factory is the Nikon 70-200mm VR f/2.8 Digital Zoom lens.
Yes
nikon 50mm 1.4 af
Yes