As others have said, you'll need money for extras when you start commuting -- lights, helmet, panniers, a good lock.
Use the hardrock for a few months and see how it goes. Then when you buy the perfect bike you've got the accessories already, and can spend all you've can spare on the bike itself. And of course a 1990s Hardrock is less likely to take a bike thief's fancy than a fancy new bike.
How far is the commute? What sort of roads?
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Zero gallons to the mile