Like jonas said, it depends on the wheels specs.
Offset doesn't matter if the wheels are like 5" wide (exaggeration).

Find your wheels, then deal with the tires later.
I personally think that 18" wheels are too big. I like 17's no more. Running big wheels is cool because it's trendy, but thats back in 2001. this is almost 2007, it wont be trendy any longer.

btw, make sure you have racing coilovers if you want to run it flush.