If you haven't already, I recommend reading through what Sheldon Brown has to say about internally geared hubs:
http://sheldonbrown.com/internal-gears.html
As far as spacing goes, if the over-locknut size is the same on your current wheel as the IGH wheel you plan to use, then you shouldn't have any problems converting. If the spacing is slightly different, and you have a steel frame, you can bend the rear triangle to make the new wheel fit.
Concerning spoke count, I'm not sure what to tell you, but I'm sure someone with experience building a wheel can help.
What size wheel are you looking to build?