There is interplay between the width of the wheel and the offset for the fitment. You can use the calculator at the link below to compare different possibilities.
myturbodiesel - tire wheel gearing calculator
For a comparison, I think 8.5" wide with a 45 offset is a good reference for a maximum combination without front poke. That is also safe for not interfering with the suspension. I favor 8" wide ET45, which is a bit more conservative.
PS: You can definitely run 18" wheels, but like with any wheel choice the center bore matters and so does the bolt pattern and type of lug bolts, e.g., Center Bore: 57.1mm, Bolt Pattern: 5x112, and Ball Seat Wheel Bolts.