Pending what size turbo you stick on it, it's sounding much closer to 400 than equal or less than 300kw.
What is the actual aim of the car? Drag street circuit etc
If I was going for a street car 300kw is well enough for a datto shell, keeping in mind they are so light.
The s15 box is the weakest of the s chassis, so option is convert a RB Z32 box, or be careful with a s14 and it will be ok.
For 300, I wouldn't even bother building an engine. Stock bottom end will be fine, but if your much over that get some better head studs. Don't rev it much over 7500, and don't bash the limiter, cool it properly and it'll be a happy reliable engine.