best bet is to put a 2nd hand RB25 in it then sell it and move on to something already rebuilt, that is if you want to stay with a Skyline.
As to where you got the price of $5k for a second hand RB25, I dunno? someone was obviously pulling your leg. When I cooked the engine in one of my skylines, the engine only cost $2.5K to replace and that was three years ago, I would expect to pay less nowadays seeing as they are so common!