I'm a UCD alum. Mathematics '81.

If you're living a couple of miles or less from campus, just about anything works. Simple has its benefits. You'll never worry about derailleur adjustment if you're riding a one speed. It's mostly flat, but you'll get wind sometimes. A couple of gears won't hurt.

Good quality used is a great idea. Get it all tuned up. Learn how to lubricate the parts that might get wet (especially chain, der if you have 'em). Fenders are nice in the winter. I never had them. I tended to walk if it was rainy and I could make the class changes. The winter gets rainy (usually). You'll do well to have a rain plan (fenders, rainwear, footwear vs. walk vs. other) Unitrans was a good alternative when I got farther from campus.

I lost one bike in my four years there. Left overnight near the MU but terminal, not locked to anything, just the cable around the frame and back wheel. Probably got tossed into a truck and taken. Lock to a sturdy object and your theft odds go way down.