Stangnet search function is the first thing you should do. This question has been asked a million times. I am not telling you this to be rude, but rather so you can see for yourself it will always be opinion as to the order of which, but all will suggest bolt ons, brakes, suspension and gears in no particular order.
Do what pleases you most. Answer one question and you will actually get a good answer and not brainless fodder; Which do you like most? 1/4 mile, corners or road course?
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