"what else will I need to have an equivalent system? thanks."
The answer depends upon what you do with K3 and what you currently like and dislike about the 756proII. The K3 has some significantly better spec's than the Icom. (Check the Elecraft website by looking at the Rig Comparisons at
http://www.elecraft.com/K2_perf.htm). The order page makes it easy to pick you rig capabilities and customize the K3. For example the filter selections depend upon what you want to do with the K3, the second receiver adds another dimension and the selection of the P3 pan adapter still more flexibility.
Tell us more and we can give your our take on the ultimate K3.
73
Dan
Dan AB3EN