Fixed your link: http://www.carsales.com.au/all-cars/private/details.aspx?R=9860548
Read this: http://www.skylinesaustralia.com/forums/topic/35129-im-buying-a-skyline-what-should-i-look-for/
Get an inspection.
Get a dyno ($80 will tell you if it's running at a higher boost, what's its kilwasps are, if it's running lean/rich, etc. etc.)
Go to a place like Unigroup Engineering, the tuners/whatever there will often know problems with the car just by looking at / hearing it.
Check the entire car for rust, parts falling apart, etc.
Take it for a drive, listen to the start up; does it take a little long? Does it SOUND healthy?
Do lights etc. dim/flicker?
Give it a hard push, don't be scared to give it a go. You're genuine about buying it, you deserve to see what it's like.
Push it high, feel for any misses / lack of power.
Come back to this thread with your findings and thoughts.
Edit: oh, and check the radio. Take a CD or aux cable/ipod just in case