Unfortunately, with the motor on its way out you would be looking at the same amount as the lowest trade in value, namely, around $11,800. You may be better off parting the car out, if you have the skills/time to do this yourself. http://www.redbook.com.au/cars/research/used/details/1995-nissan-skyline-bcnr33-111628?R=111628&Silo=spec&Vertical=car&Ridx=9&eapi=2 To get the thing back to where it needs to be with engine rebuild and other repairs would be around the 15k mark as a rough estimate depending on how bad it is.
Another benchmark car is a 1996 bright green skyline GTR V-Spec with all the fruit, but some interior and bits missing for $11k. It has been on the market unsold for quite a while http://www.gumtree.com.au/s-ad/mount-warren-park/cars-vans-utes/1996-r33-gtr-nissan-skyline-v-spec-repairable-write-off/1031209389