I would strongly suggest avoiding Nistune with an R33. There are numerous other ECU's that will plug and play such as PoweFC (older tech) through to Adaptronic, Haltech, Link.
I would consider carefully what you want from the car as careful selection of parts in the first place will save you thousands. If you want more than 200rwkw then you are into the world of aftermarket turbos and new injectors. Each change will require more tuning so take your time to figure out realistically how you will use the car and how much power is needed to achieve it.