to be honest 19s or 20s are the best fitment looks wise (they are heavy and all that but just visually speaking) 18s are to small overall~ the only way (in my opinion) to pull off 18s correctly on a 05-09 mustang is to use a tire whos sidewall is shorter than stock (use the fitment of a 99-04 mustang) this will drop the sidewall about .25 inches and make the rim look bigger. The HUGE sidewall of the tires throw off the rim size and make them look smaller.
You run into an issue with the speedometer by doing this and will need a tuner to calibrate it~ thats the only hassle for the most part.
As far as fitment width wise you should be able to fit up to an 11 inch rim in the back and a 315 tire.
BE VERY CAREFUL on the rim fitment specs you look at~ review my article here about fitment
Mustang Wheels and Tires FAQ
read rim fitment overview section it will answer how you want the rims to fit
I dont have the fender flush information on 05+ but I can get it (I have it currently listed for 94-04...) but this will get you started in what numbers mean what on rims.
I am assuming your rear front spacing needs to be 3 inches going by how my 03 cobra rims look on my friends 08 gt.