KezR33
My words of wisdom.
If you can afford the repayments, you're smart with money, can afford the maintaince and have solid job security, go for it. It can build a solid credit rating should you pay the repayments everytime.
Also like someone said, better save up a bit more for emergency repairs or expense (excess, rego and damage that replacing said damage is lower than the insurance excess).
If you're familiar to the maintance of Skyline, you should have some idea of maintaing Silvia or Supra.
If i were you, I'd wait until you're off your P's and choice of cars will open up a lot (could pick 34 GT-T for same money as S15 nowdays), just a thought also perfect excuse to save up more cash for mods.
All the best with the decision.
Cheers