Hi Leah!
Congratulations on starting into the world of cycling! I'll take the first crack at your questions, and the more experienced among us will correct me.
Is a WSD bike better in any way than a unisex?
It definately depends on your build. WSD bikes tend to have shorter grip-distances for the brakes, among other things.
Is it worth it to look for a used bike and where are good places to look?
Used bikes can definately be worth while, but be very careful to inspect the frames for cracks, and make sure all the components are in good shape, or count their replacement/upgrade into the overall cost. A lot of grand old ladies of the bike world are very worth repair and have many, many miles in them yet.
what are some features that I should look for in the frame?
This is best left for our experienced ladies...
are there any makes or models that I should stay away from?
You mentioned taking your bike to college with you.... I'd stay away from taking a *new* or *good* bike to school. Sadly, lots of people who go to college are still just big kids and will trash or steal someone's perfectly nice bike. I'd take an old beater or if need be just a GoodWill bike tweaked into good performance.
As far as the Nishiki, I'd have an LBS take a look at the brake pads, cable wear and tensioning, etc, or at least go to parktool.com and have at it yourself if you're so mechanically inclined. There's just no excuse for unsafe brakes. If the bike is truly *that* old, it might need new pads.