Yes and yes the RB series from a skyline will fit in either the Rb20 RB25 or Rb26. Also you don't have to swap your motor if you don't want the KA that you have now is great for turboing. The S13 front will fit on your car. There is bunch of cool stuff for your car i'd dig around on a few sites for parts like
http://www.raretrick.com or
http://www.jdmparts.com