That sounds like a good decision. My "beater bike" is a 1993 Giant Option. It's actually a great bike and loads of fun to ride. It's nothing fancy: a steel frame, non-suspension fork (I hate suspension forks on inexpensive bikes!), 10-speed that takes racks and fenders. It gets me where I need to be, carries all my stuff, and doesn't attract the attention of thieves. For your relatively short commute, an old hybrid or hardtail mountain bike should do very nicely. Good luck!