Save some money for accessories you'll need -- lights, rack, fenders, pump, tools, and more.
For your own safety, you have to be visible -- that's why lights (blinkies)
Fenders -- after riding just a mile on wet surface (even AFTER RAIN) -- your back and your pants/legs will be wet.
Rack -- yes, you can carry your stuff in your backpack, but that will make your back soaked in sweat. Sometimes might cause pain in your sholders.
Tools -- you'll need to adjust your bike (saddle highth or tilt) in the middle of your trip. And you'll have to be ready to fix your flat tire.