you can put 18s on your celica without any suspension mods. (to answer your original question).
you can even put 19s--but i wouldn't recommend it.
however, when putting 18" & larger rims on, be VERY consistent with maintenance. your tranny will have to work a LOT harder to spin your much larger rims and therefore if you don't take proper care of your drivetrain your tranny will suffer quickly.
with 18s you should be able to drop your car up to 1.5" to 2" (depending on your tire) and still be able to drive around decently.