So I really want a road bike, but some things have come up (need a laptop for school, AC broke, etc), so that's not going to happen anytime in the very-near future. So I was wondering if building a SS with the idea of adding a cassette, derailleurs, and shifters later on is viable...
Sure, you can. Buy everything but the shifters, derailleurs, and cassette, and set it up with a chain tensioner and a single sprocket with spacers on the rear hub. Of course, you will need to buy a different set of brake levers (assuming you're going with integrated brake-shift levers for the road bike), but as long as you are going SS, a tensioner will get you around the dropout issue.
I'm not too familiar with that frame, so I can't comment. What gear ratio you use depends a lot on the terrain and your level of fitness. The late Sheldon Brown's gear calculator (www.sheldonbrown.com) can help you find a good starting point. 70-75 gear inches would be good with hills IMO, and that is pretty easy to get with a 53 (or 50, if you choose a compact crankset) and a fairly large cog.
Sheldon's site also has a few pages on setting up a single speed, too, if you have any more questions about using a tensioner or spacers.