Welcome!
Quite a few questions you have there. Here are some responses:
Skylines are obviously Japanese for a start, so I would say they would be more durable than anything built in Australia. Nissan's generally are very well built... our family had a bluebird that did over 500,000km despite not being very well looked after for the second half of its life!
The reason you don't see many with over 180,000km's is two fold. In Japan cars generally do not do the same mileage as in Australia, also MANY have their odometers wound back before import.
I am not familiar with the gearbox's in gts-t's being a problem unless you start putting too much power through them, but that is not really a surprise.
A GTR 'bodykit' would mean the owner would have to have purchased, fitted and sprayed the following:
GTR front fenders (aluminum)
GTR front bar
GTR grill
GTR bonnet (aluminum)
GTR rear fenders
GTR spoiler
As you might appreciate these parts are not cheap, so I would not hold my breath... At any rate, please do not put a GTR badge on your gts-t unless you want instant looser status.
Hope that helps, and goodluck in your search.