Lucky for me, ITBs come standard ;) ITBs is the ultimate step for NA tuning (Well it can be applied for force induction but lets not go there ;)) you pretty much do it last, its costly, if you're actually looking for power gains, you're looking in the wrong place (NA gains power/torque either via cams or via displacement if all else is kept equal). Do everything else to the engine forged internals, (forged internals if you're planning on revving out to stratospheric heights) machine work (port n polish, combustion chamber adjustment, blueprint, balance, etc etc) and decide on where you want your powerband to be which will dictate what cams are needed and if your valvetrain needs to be strengthened. Also if you're fuel system is reaching its limits then may have to look within the family (Supra, Chaser, Celica GT-Four) for bigger injectors and fuel pump. Then as mentioned an ECU that can handle it, most likely a standalone needs to be used. Its just a general plan above cause I'm not too familiar with the exact specs of the 2ZZ and the auxiliary components around it.