Your cruiser style 3-speed is fine for a 3-mile commute but you may prefer something speedier and more efficient for a 2x7 miler. Avoid anything too flashy, you need something that rides much better than it looks.
Bolt-on rack and fenders will be useful. Std hybrid bikes are the most common in used market, so are cheaper. Try and avoid suspension forks.
Don't go cheap on tyres, they get you to college/work on time. Schwalbe Marathon work for me, but marathon plus are even more proof.