What can you tell us about your existing hub? If the cups and cones aren't pitted I'd probably just cut the old spokes and lace a new rim onto it. If you can see any pitting on the cones AT ALL I'd dump your hub in favor of a new one. Building a wheel costs too much time and money to not to end up with something that you can be happy with.

I'd agree with the Sun CR-18 rim choice. I use spoke calc to determine spoke lengths. For a project like this I'd settle for straight 14 ga stainless spokes.