If you already have a turbo, you will probably need, to get the most performance out of it. Fuel upgrades (injectors rails pump) Air flow upgrades (intake, exhaust, maybe a higher flow maf (depends on car)) engine management system. you need to program the system.
Yeah i only have an intake and an exhaust...thinking about getting headers soon...and i do not have a turbo yet...so im thinking might have to get cams shafts,cam gears and like valve springs
a head job is probably a good idea but if you have low miles it may not be necessary. And you may not see so much performance per dollar also might affect drive ability if you put "high performance". If you really intend on turboing I would save for the required items and not buy say, the headers, then have to go buy turbo headers or custom fabricate them. I hope you understand. Also I am no expert in this.
you might wanna search kia forums for an already turbo and check out what and where he got his upgrades, fuel, intake, exhaust, manifold, and engine management. See whats out their available for your car.