get a serious entry level mountain bike like a kona caldera and put slicks on it. lockout suspension is a plus too.

or if you're really handy, get an older used mountain bike and swap it to a rigid front disc fork.

stopping power is very important in a city. so is being able to take potholes and curbs with ease day in and day out.

for that you need wide tires, robust frame, and good brakes in good repair.

spend as much as you can but budget in a helmet, a set of lights and a crazy nice lock for the frame and thick cable for the wheels.

i rode with a guy last night that had an on*one il pompino with a 29er front disc fork set up singlespeed and clipless. he had road tires on it but the wheels were bombproof. that particular setup is probably spendier than what you have in mind, but a fred'd out sora equipped road bike with bright flashy paint fresh out of a store is going to attract thieves no matter what.

i think a 26" with slicks and discs would be the best bike possible. rack is a plus.