Hey, Christian
You can go up to a set of 9"s & 10"s without issue in the proper tire size and off-set. I run a set of 1/2" wheel spacers out back, but have replaced my lug-bolts with longer/stronger ones to do so. There are lots of them out there that have a second set of bolts built-in to avoid this issue, but not a fan of them. I recommend you just get wheels that fit your ride the way you would like and forgo any spacers.
You can run a set of 20"s, but will be VERY limited in the tires you can run, so I think you are best to avoid them. I believe an 18" wheel to be perfect for ALL '94+ Mustangs and will have many, many good choices in tire quality as well as sizes.
Take a peak at Tire Racks Decision Guide in Choosing a new tire thread of Tires/Wheels main page to help figure out what would work best for you and be within your budget.
Jazzer
