first thing i would do with an import RB26 would be to strip it down and check the internals,if alls good give it a hone,linish the crank,new set of rings and bearings,new oil pump,pressure test the head,check valve seats,throw in a set of valve stem seals,check the valve springs carefully for tension,install a new set of retainers,new set of gaskets and you are good to go
the rb26 is a very expensive handgrenade and i have lost count of import ones that have met with premature demise after just being installed as is
....particularly spun bearings or dropped valves
i'd budget on a minimum of $10,000 ,including ECU & tune,loom,fuel system,wastegate,600hp turbo/s,pipework,exaust,engine freshen up,clutch,tailshaft,injectors,custom sump/pickup
however the above may not guarantee a reliable 600hp,for this you may need forged pistons and rods,large snout crank/mod,N1 oil pump,HD valve springs,aftermarket cams
hope you've got deep pockets