I went from a VS SS to my Astra turbo a bit over 3 years ago and could never go back to driving one. For mods you have a few choices, basically you need a intercooler and exhaust if you plan on increasing the boost, you will also need to remap the car. Probably looking at 3k to get you at that point. Will up your power from 200hp to 250hp at the flywheel.
From there you can get a vxr turbo unit and vxr air flow metre and injectors, that will take you closer to 300hp and basically be the limit of the stock engine internals. Probably another $1500 to do that if you are handy with the spanners.
For handling most people go coilovers or an eibach / bilstein spring shock combo and a white line rear anti roll Bar. The stock brakes are fine unless you are tracking the car.
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