The stock size of 195/70 X 14 is a good match to the motor and size of the wheel wells. The original 240z came with 175 width tires! If you go any wider on the front, it will be a harder to steer, and bigger tires on the rear will hit the wheel wells. I know, because I had a 240z with 225 width tires on it. If I resized, I would buy a 195/60 X15, same overall tire diameter, but a shorter sidewall should sharpen up the handling. My brother did that on his 280zx, and he was happy with the change.
I wonder if anyone has tried mounting different width tires and done any real testing to determine the effect on cornering ability, either by checking lap times or using a g-meter. I don't think I would chop up my wheel wells to add flares just because I thought wide tires looked cool.
The stock aluminum rims are pretty common, I have two extra sets in my shop, and paid $50 for a set of four. As far as tire width, most wheel manufacturers say to stay within 1" of the rim width. For a 7" rim the tread width should be between 6 to 8 inches.