Buy nice R31 GTS R, apart from the R32 GTR that would be a nice Skyline to hold on to with the view of making a few bucks in the future, or at least not lose 50% of the cost price.
Much safer bet if you ask me, there may be a demand now hence the inflated prices, what happens in 5 or 10 years, I wouldn't buy a 34 as a investment, yes its the most advanced Skyline GTR but being the most advanced or last model doesn't mean its a worth while investment, a car needs history.
Ask any modern fan boys about their cars, Honda S200, Toyota GT4, Mazda RX7, Subaru 22B, Falcondoore VLXRSS8, every one says their cars will be collectable.
Anyways, keep up the good work.
Disclaimer: I could never keep a car nice enough to be classed as investment quality, some smart bugger once said something like, "Not driving your car is like not sleeping with your girl so she is more desirable to the next guy".
LOL, cars as investments, good luck with that.