This is the kit I bought a few years ago Koni Struts/shocks and steeda springs. Strano Performance Parts - Part Details
I did go ahead and get GT500 strut mounts(stiffer compound I used with the j&m caster/camber plates) 2010-2014 Mustang Caster Camber Plates at AmericanMuscle.com - Free Shipping!
I also used UPR double adjustable upper control arm and J&M adjustable panhard bar 2010-2014 Mustang Panhard Bars at AmericanMuscle.com - Free Shipping!
as well as a BMR relocation bracket for my J&M lower control arm
If you are just concentrating on lowering you may not require all of this, I'd get the panhard bar for sure and then the stranosports kit of your choice if you choose to go that route. the GT500 strut mounts have a stronger cushion(insert) that is why I used them
If you lower more then 1.5 in the back, you should consider an adjustable upper control arm to correct any driveshaft issues.