GT+T's are quite reliable and strong if you have a good enough tune..
A few wise words in regards to ECU..
I have gone with the Apexi Power FC, you will have problems with your VCT and the intake manifold valve as there is no way to control it on the ecu.. VCT pinout needs to be altered and a rpm switch is required to get the intake manifold working.. Both of which i am currently working on.. Intake valve can just be set to open (basically the same as a gtt) and vct is just unplugged.. They can be fixed with a few hours work but i havnt had the money to do it yet.
I am running, r34 highflow, 600cc injectors, power fc, FMIC, full 3" exhaust, Blitz EBC, splitfire coilpacks at 18PSI through a rebuilt manual box (260rwkw) and have had no dramas with it at all.. Total cost in parts is about 7.5K including box rebuilt (add another 1k for clutch if you are running stock)
I have done all the labour myself so cannot comment on how much it will cost.. Roughly 2k depending on what needs to be made up to suit. You cannot run the standard GT airbox if you use the factory intercooler piping. Just get a good pod and make a heatshield and you wont have probs.
Yeah not cheap if you go with quality gear. Most of these you will upgrade in the long run anyway so do it once and do it right. instead of wasting hundreds on standard stuff only to bin it later on.
I could say alot more but cbf atm.