also i would like to know what the best size of rim i should purchase!
i was thinking 18x9 in the front and 18x10 in the back, would i have any probs with that such as my tires hittin my fender or anything else??
18x9" for the front and 18x10" for the rear will work fine as a combo. A 5.9" backspace on the 18x9" front and a 6.3" backspace on the 18x10" rears will fine. That's what most of the aftermarket Mustang wheels are spaced at in the 18" sizes (FR500s, Bullitts, etc.).
You shouldn't have any tire issues using wheels with those backspacings. You may however run into problems if you try to go wider than 315 on the rear, but you can probably compensate with a wheel spacer if you need a tire that wide.
new edge BLACK SALEEN S STYLE wheel's (w/o rivets) look great on Mach's, especially on your DSG. i have a 03 TR Mach and just got the black saleens from american muscle, they look amazing. i went with 18x9 in the front and 18x10 in the rear. as far as tires go, i went with nitto 555's, 245/40/18 in the front and 285/40/18 in the rear and they look and ride great.