Firstly congrats on the car. Those are rare to find, I hope you have many good years with that car.
Secondly I think the car looks pretty good stock. I would the windows darkly tinted. This is one of the least expensive things you can do to your car that will make it look dramatically better.
Then I would get Halo Ring Projector Beam Headlights.
Here are some examples:
http://www.carid.com/2005-toyota-celica-headlights/
Make sure you also get the HID kit for the headlights.
Lastly I would get LED taillights.
Here are some LED taillights:
http://www.carid.com/2005-toyota-celica-taillights/
I like the red LED lights.
Stay away from body kits or any other major cosmetic modifications. This will decrease the resale value of the car.
Anyways good luck!