My personal opinion in this: if you want outright moddability, the R33 is the way to go. Not only is it the most common model on Australian roads, but its pretty much the best supported Skyline in both Australia and Japan.
I personally prefer the stock R33 GTS-t looks over the stock R34 GT-T looks: something about the GT-Ts doesn't quite float my boat. Having said that, I think the R34 GT-R is the nicest design of all, and some of the GT-T kits do wonders for its appearance.
Taking into consideration its a newer car with improvements, I think the R34 GT-T is the best compromise between price, (stock) performance, moddability, and 'new-ness'. I would only have one priviso: buy a manual if you buy a GT-T so you don't have issues with aftermarket ECUs.
LW.