I wont talk on the "look upgrades" since that is subjective to each personal likes, however in the performance area you can do the basics, air intake, exhaust, headers and if you have lets say an extra $1000 in budget go camshafts.
You can use a car performance simulator to virtually tune your car and run performance test to see how the car would improve in real life, try at
http://www.nxgtrsim.com