Your options our as big as your wallet. You can upgrade anything if you have the money. You need to decide what exactly you want to do and what you are going to use the car for. Will it be driven on the street or on the track or both. Once you decide what you want to do figure out a budget so you know how much you are going to spend. Then you can start looking for the parts you want. The main thing is to have a plan. Once you have a plan you can change the plan to suit you needs and adjust it as you go.
Have a good one,
Bill