Tires: Continental ExtremeContact DW are my.goto. Cheap, grippy, comfortable. Perfect DD summer tire. If you want to prioritize comfort, stay with 16s. If you want more steering response at the cost of some comfort and price, 17. As for wheels, Fast and Tire Rack have plenty of selection and nice preview tools, but avoid heavy wheels (over 19 pounds) as they will kill steering response.
The Nut: The nut in front of the steering wheel is you! By taking advanced driver training you will be able to see farther, brake harder, turn faster, drive smoother and more, all for much less than a collection of suspension mods. Suspension mods will increase your car's abilities but unless you are pro you will not be anywhere near the stock car's abilities.
I'm not talking Young Drivers or something. I'm talking learning to handle your car at its limits, and learning where they are. I highly recommend
www.carcontrolschool.com as the instructors are all from racing backgrounds and you learn in your own car.
As for autocross, I recommend it not to compete to win, but to compete against yourself and sharpen your own
skills. It's excellent vision and at-the-limit practice while being safe and very cheap.