with that budget you should ask yourself what YOU want cause you can pretty much get anything r31 wize, with a few exceptions, gts1 or 2 should set you back a bit more than 7-15000. i'd say look for a looked after Silhouette ($2500-3000) and spend the rest on suspension,leads, plugs, pod filter, extractors and exhaust...
an example for you, this one has had some work done
http://forum.r31skylineclub.com/index.php?topic=31978.0