XS kit should be a decent one but it is for a GTS and will not go well on high boost, our stock MAF sensor sucks it will send crazy signal to the ECU when going past 5psi boost (why most kits are at 5psi wastgate setting). Though XS-Engineering kit will come with PowerFC preprogrammed, it will not work for high boost either because it uses stock sensors and our sensor is the problem.
Now taking turbo from older model Celica, you do have to do a custom manifold pippings. Our space for turbo manifold is way too limited (engine space is too small). You can use the turbo charger (if small enough to fit back there (should not be a problem), intercooler (if it has one) but thats about it. Add on parts is as follows: Pipping, manifold, downpipe, straightpipe, management system (for people who want to run above 5psi, and must have a replacement for MAF sensor or send stock signal to deceive stock ECU), oil pressure gauge, Emission Gas Temp gauge, boost controller, Fuel Pressure Regulator, Return Fuel Line, Bigger Injectors, and bigger fuel pump.
GTS require building bottom end of the engine.
Options: Boost gauge, Blow Off Valve.
These are essential parts to run right and safe, now I may have left out some things but I doubt it.
Its not a small project, should give it to someone who has modified a Celica before.