u dun reli need to upgrade ur brakes, u could just get new na rotors n pads n use them (as the current rotors u have r probz around 15 years old)
it shouldnt have cost u that much all up, on average the rb25det engine cost 2.5k n at most should cost another grand at most to take out ur engine n drop in n hook up the loom of the new one
u can also keep the diff, but just shim it n it should b all good
ur clutch wont b able to handle the new power so u will needa heavey duty one
to the dude above who sed its easier to sell his current skyline n buy another one, just a question for u, have u checked out the for sale section in this forum for whole cars? well theres alot of sweet ass skylines there that have been on the sellers market for a long time.