The R34GTT is a super reliable car in stock form, i drove mine with mild mods for years without a single drama..... I purchased it back in 2008 (I think, i cant remember) and have taken it from semi stock form to the animal it is today.
In terms of fuel consumtion you should get much better than the supra in manual form! Simple mods can gain you power, once you chase more you will need to upgrade all the supporting parts which costs a bit more than the R33 but its still overall a better car with newer technology and as you mention, more power/torque.
If you are spending $15K you can get a good quality example with some mods already done.
Look up the skyline buying guide on SAU which will help you and also get a proper inspection.
I think youll love it!