Okay let me try and answer your questions:
1) is it okay if i change Fx 6300 to 8350? Yes you can, but not with the Mobo I listed. If you jump up to the 8350, I would recommend a different Mobo to power that CPU (it requires better/cleaner power for it).
2) or the i5 4590? Yes you can, but you would need an Intel Mobo for that CPU.
3) and does dis fit r9 280x into NZXT GUARDIAN 921 RB? It might, but I'd have to see the dimensions closer. I would opt for the Corsair 100R/200R, if they were available (they would fit just about any GPU out).
4) RAM and HDD are good options.
5) SSD is my preference. It isn't need, but I prefer it. They are spendy and not necessary, but that will be up to you and your budget. I would rather get a better CPU/GPU then spend the $ on an SSD (when the budget is tight).