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Always get quality oils and change them every 5000km
Bit of injector cleaner can be handy every 10000km or so
Get throttle body and parts cleaner to clean your spark plugs and throttle body combine...

Depends where from, what brand, if it's custom, what your budget is etc it's a hard question to answer.
I mean for headers alone you can spend like $150 on some cheapy dc sports ebay headers, or all...

Oh almost forgot, SWAY BARS GET SOME SWAY BARS ASAP BEFORE ANYTHING.

A good cold air intake with a nice pod filter and a good full exhaust system (headers, catback, axelback) will net a good amount of power for the cost and make the car sound 10 times better.

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