you can get gt and cobra take offs for cheap... say 200-400 depending on the style and size.
personally, fords (mustangs) handle like pigs and take a lot to handle for anything. if you're looking at it strictly for performance of the car, 16x7.5 (what you have) is definitely good enough. will you see a small improvement with 17's? yes. however, with what you have, i'd recommend suspension upgrades first. might as well make it all you can be before you spend money on wheels/tires, right?