If you're not after anything too serious then start with a full inspection of the car and a fluid change. I'd also recommend a compression test before purchase.
The car might even come with a decent clutch if you're lucky.
I'd recommend swapping out the ceramic turbo's as soon as you can as they're known to fail. 707160-7(R34 N1) or the HKS varient GT-SS is the next step up from factory and will give you roughly 400hp to the wheels and a high 11 second 1/4 with supporting mods.
Supporting mods would be a full exhaust, fuel pump, injectors and a tune to reach that goal.
Search will be your friend as to which way to go about it.