Based on my ownership experience long ago with one of these (1983 - 2000) ...
These cars are extremely sensitive to wheel/tire balance. Find a really good tire shop to properly balance them, see if that helps. Also, after you have the balance correct, slight tire pressure adjustments such as having the fronts 1 or 2 psi more (or less) than rears will help. But you'll have to experiment to find what works for you.
And are you sure about an 18 inch wheel? Stock was 13, I'm not sure that 18 will fit. If so, what other modifications have been done, and how would that impact the suspension?
You said that the wheels have the correct offset. Are you using spacers? If so, make sure that the spacers are hub-centric (properly align themselves to the center of the hub.) This will also cause the steering wheel shake.