Tires and brakes are probably your best bet. Tires will make the biggest difference. Most people don't realize just how much tires do until the get a really good set of performance tires.
Brake pads are another thing to look into. The last thing you want is brake fade after 10 minutes of lapping.
And watch your engine and fluid temps. Then you can see how well your car breathes.
Knowing a good photographer can be fun too.
