Jin,
Buy a box of fuji first. Kodak endura is the best paper but as others have pointed out it comes in rolls and needs to be cut down by hand. Once you are comfortable, buy kodak endura.
The chemicals can be reused. Kodak chemicals keep very well. Over time they will become exhausted and need t be 'replenished' in the case of RA-4 or replaced. Make sure to use a stop bath. RA-4 does not require one officially but it is highly recommended to use a 1-2% acetic acid bath.
No you cannot use the chemicals interchangeably. Technically you can, but the results will not be good. The developers use CD-3 (RA-4) and CD-4 (C41) at different strengths which will not give you what you expect. The bleach is also a different strength.
Enjoy printing and don't be shy to ask more questions.