generic cooler kits, turbo kits, injectors, universal fuel pumps, coilovers, ecu upgrades, wheels, gauges, exhaust systems..... i cant really see it a problem getting any of these to suit a R34 GT-t.
If you buy Japanese, or name brand, you shouldnt have any issues with finding stuff.
Greddy (Trust)/Blitz cooler kits
bolt on Garrett turbo upgrades to suit stock manifold or a Greddy/Trust kit.
Injectors (Nismo/Sard etc)
Bosch/Walbro/Nismo fuel pumps
Tein/HKS/G4 coilovers
Power FC/Haltech/Microtech/Wolf/Autronic ecu ugrades
Wheels, anything Jap like Volk/Rays or a GTR wheel
Gauges, (Blitz/HKS/Defi/Greddy/Autometer)
Exhaust, maybe X-Force does a bolt on kit, otherwise something Japanese.
Would be as easy to mod a R34 as it would any Skyline Id say, especially if your willing to buy brand new. Obviously the second hand market wont be any where near as strong as the R32/R33 market as the R34 hasnt been around as long.