One thing to consider is rotation. If you put bigger wheels/tires on the back of the car then you can not rotate your tires. This may be an issue in tire wear. Just a word of warning if you are to consider this.
I have 18x9 in the front and 18x10 in the rear with 275/35/18 tire in the front and 295/35/18 tires in the rear. You should go no smaller (the first number is the width) on a 9 inch and 10 inch rim respectively. There is no point in buying wide rims unless you plan to us them (aka use wider tires).
If you want to maintain the ability to rotate you can run 18x9's all around with 275/235/18s (you can go 265/35/18s if you have to but no smaller).
Mustang Parts & Performance - American Muscle
has a ton of wheel/tire packages and since you are a member get with use for a 6% off discount!
If you want staggered wheels (front different size than the back) then I can defiantly help you out. You can click my wheels in my signature to see the setup and fitment that mine have.
I think 18 inch rims look best on 94-04 mustangs. The key thing is lowering the car, a lowered mustang with 18 inch rims looks best.
read this: http://www.mustangevolution.com/articles/2009181/
it will answer many questions for you