I know it’s your car and all and I don’t know how you’ve modded it but if you have not done suspension, you should get on that before anything else. Lowering springs don’t count.
If you have already done suspension and tires, then ignore that.
Also, with 300whp, I hope that your brakes are somewhat upgraded, at least 180sx brakes with HAWK HP+ pads, but even that is kinda weak for that kind of power.
If you pretty much just want 300+whp, i’d suggest doing this.
S15 Turbo (Try to get one brand new)
550cc Injectors (Nismo, Tomei, Sard, anything brand name)
Z32 MAF (Make sure you find one with the pigtail still intact)
Exedy Stage 2 / ACT Heavy duty clutch or w/e / or any clutch that will handle above 300tq.
Engine management (I guess you already have your EMS)
3 inch full catback exhaust
Cooling might be an issue so I’d suggest getting a FMIC
An upgraded radiator would help too, a KOYO is the best bang for your buck.
And when going with the S15 turbo, if you want to retain your hotpipe position and turbo inlet pipe the same, invest in a set of those S15 Turbo adapters. If you put those onto the s15 turbo, everything will line up just like stock. 
By the way, if you’re just running stock boost, why dont you just turn it up to 10 - 12 psi, it’d be a lot more fun that way.
Good luck.