If you wanna make 300kw go with a TD06 L2 20G or a GT3037. You might be able to make this on stock internals. You'll still need an ecu, injectors, fuel pump, and a boost controller. Cams would be a good idea too. I think the stock headgasket is fairly strong on an rb25.
450-500kw and you'll need a few more things. Aftermarket transmission, bigger injectors, forged crankshaft, probably a bigger intercooler, boost controller, radiator and oil cooler for safety, and probably 1000 other things I'm forgetting.
My example is using all new tomei parts. I'm sure it could be done cheaper using used parts. I'm not an expert in doing things the cheap way. I always like to buy everything new when possible.
Do not use a T78 to make 350kw. This turbo is way too laggy and you'll have a horrible power band for only 350kw.
Rough estimate using all new Tomei stuff:
Prices USD$
Pistons 1,400
Rods 1,300
260° Cams w/pullies 850
555cc Injectors 780
1.2mm Headgasket 280
Apexi Power FC 1,200
Then you still have to add labor.
What mods does your car currently have?
You don't need that big a turbo to make 350kw! T78 is a huge turbo 500+kw. You could go with a GT3040, rated for 375kw, or T67 25G, rated for 410kw. What is your desired budget?