Like I said, what gear combination do you ride in most? Set your bike up so that's the center of your cluster, with whatever chainwheel you decide on using. That way you'll have what you're comfortable with, plus two up and two down.

On the internal gear option, do pay attention to the ratios that a given mechanism will provide, and take that into account when deciding which to use, and what size sprocket to fit it with -- factoring in chainwheel size too. There's another thread running where the S-A 8-speed unit was mentioned. Harris sells this for $170 with the shifter and a 25-tooth sprocket. However the lowest gear is direct drive, and all the others are higher! So you would want to use that hub with a really small chainwheel to get gears that are lower than what a 25-tooth sprocket would give you on a derailer bike -- like a 34 or 35 tooth chainwheel. That would mean getting a new crank in your case.

The traditional S-A equipped 3-speeds were geared rather high, with a 46 tooth chainwheel, and about an 18 tooth sprocket. Middle gear is direct drive on those (as it is on the old 5-speed, I believe). You can get sprockets for them up to 22 or 23, if I remember correctly. Harris sells them. Also, a S-A sprocket needs a 1/8" chain, but Shimano also makes them that will work with a 3/32" chain (typical on a "road" derailer bike). No difference in cost, just another thing to buy!

Oh, and having a correct chainline is much more important with a non-derailer rear end -- either internal hub or single speed. The derailer keeps the chain from popping off the rear cog at inopportune times, and the bike will be more efficient and silent if the chainline is right.