It's a 1.5" drop on both front and back. Yes, it's lower than it ever was.... first, avoid potholes as much as possible. 2nd, get the Eibach Pro-Damper set which are the struts and shocks. They work with their pro kit springs and make a huge difference. I was surprised by the ride difference with those. It's not soft, but hitting a crack wont be like a small speed bump. It's not a perfect ride.. you lower it, there will be consequences. But ride quality is the price you pay for the performance and handling gains. There's not really much else I can tell you. If you are afraid of scraping something on the ground for it being so low.. don't worry too much. It's not THAT bad.
Justin, You should have no problem with 18 inch rims. Those wheel wells are much deeper than the 79-93 Mustangs and can fit much larger a tire/rim. I would first lower it, see how it fits with the wheels you have on then go from there. Check out all the others rides on here. Lots of lowered stangs with rims. BTW, how wide of a tire are you planning on going?