Just put Steeda Sports on my 2013 GT, and I am very pleased with them. They are great for a DD and IMHO, lowers the car just the right amount. FWIW, I'm running them along with Tokico D-Specs and GT500 strut mounts. So far it's been a great combo, and it doesn't break the bank.
That being said, I have had Eibach Pros (springs, dampers) on a couple of other Mustangs in the past, including my '07 GT/CS, and that was a great set up as well. That combo was a bit rougher on the street, but it was great for autocrossing though. And it also was very affordable.
I'd say for your budget, go with a good spring/strut/shock package. There are many good ones out there, and you'll get many different opinions. Also keep in mind that if you go lower than about 1.5" or so, you'll need to add in the cost of an adjustable rear panhard bar to center the axle housing under the chassis.