I would suggest 17s because tires for 18s are outrageous. Depends on what kind of look you want and how deep your pockets are.
Usually Mustangs go with 17x8, 17x9, 18x8, or 18x9. They do make wheels wider than 8 and 9, but that will most likely be too wide and too much tire for your V6. I know all this cuz I just asked a question about 17x10.5
side note, i have 17x9.
they do make wheels that are "import wheels" (i.e. thin, like the 17x7 you mentioned) that have the correct lug pattern and offset, but imho, they don't look good. They also won't be as good for handling and what not.