A hardtail mountain bike (front suspension ok, no rear suspension) with slicks will make you a fine commuter bike. You would not want anything with a full suspension as it adds alot of extra weight and if the shocks don't lock out, then you would have alot of forward momentum and energy being sucked up by the suspension and it would just wear you out going nowheres. Those are dedicated to serious off roading and rocky, rooty trails. Also to consider would be Hybrid bike which is more inbetween the hardtail and a road bike.

With your budget you should be able to find something good along those lines. Obviously there are even better choices out there like the European Treking bikes that I just learned about more recently and standard commuter bikes available at LBS but those require a little more coin. If your capable of doing your own repairs, you will do just fine and spend all your gas savings on upgrades. LOL.