Well for your question about the fascia, all you need is a mounting bracket for the right side, and just lower the pipe a tad so that the tips go under the bumper, since there are no cutouts. It doesn't look as bad as you may think..
btw: 18x9 front 18x10 back is the best thing in the world on a mustang..especially if ya got it lowered.
And if you plan to do drag racing, the 20" rims will make a difference