I think if you go used, don't limit yourself to something specific. You might not have heard of the brands, but they might be decent. Also, don't limit yourself on the price too much - most of the time you can haggle.. play on your poor college student status might be helpful

Definitely go for test rides. If you know somebody experienced, ask him/her to come with you to see how well the bike fits you - important.

Of course you have to do your research as well. Know your components.. make sure you check out the wear and tear because they can be expensive to replace.. definitely try to shift thru all the gears, check if there is any noise from anywhere on the bike.