welcome along,
you really need to decide what you would like out of the car. Then check to make sure the car is still in great health.
As for Carrera2RS stating not worth it on a 75K old motor. In all honesty I can see where he is coming from. But if it has been well maintained and continues to be you should have no issues. Our mk4 is now approaching 200000km's It's been boosting 21psi since 80000km's. It's still going strong. But again it's all in the maintenance.
If you want to keep this car then go right ahead.
Otherwise sell it and get an Edition 30 or Pirelli. They already have K04. Plus a stronger motor, So mod for mod they make more power.
Personally I would not bother with a K04 on standard GTI, well over here in NZ anyway. As the cost for K04 or going all out to the likes of an APR Stage 3 or similar kit is less than $4000 difference of the get go. Plus the APR kit already has the fuel pump, injectors and Intercooler. So end cost for me over here is almost the same.
However if you were driving an MK6 GTI, then I would strongly recommend/suggest going K04 (only GTI, not ED35 or Golf R). You will end up with 360HP out of the box with the likes of an APR K04 kit or a whopping 460HP out of their new Stage 3 turbo kit.
The Edition 35 and Golf R, both still have the older TFSI engine, so camshaft bucket issue, fuel pump upgrade etc. More maintenance.
Hence my first question was what you would like to achieve from the car. As this will point in the direction you would need to go.