I commute 6.5 miles to work each day, and I'm usually racing to get there on time. Unless you are slogging through snow, you might think of eventually getting a light bike with a rack for paniers, and more narrow tires. Panniers allow you to cary workclothes, lunch, lawn darts, ect. Most of us like to have lights for when it's dark. All this can cost a bit, but it's nothing compared to daily public transit, or owning a car. If you begin bike commuting, you will soon be much richer than your coworkers who drive. I suspect you'll be happier and more fit as well, but that's just accidental.
Anyway, start riding whatever you can afford, and eventually, you'll have the cash to buy whatever rig you want.