Adjustable Panhard Bar, in which I recommend our PHR006:
BMR-PHR006, S197 Mustang On-Car Adjustable Panhard Bar
The only other recommendations I have are:
Camber-Bolts or Camber-Plates, to dial in your camber, IF you choose to do so. I typically recommend waiting to do that at the end, a LOT of my customers choose to run the negative camber that they get from the springs.
LCA Relocation brackets will help restore and improve upon the LCA Geometry that is negatively affected from lowering the chassis of the car. Of course, with the BMR set, you are getting the best "bolt-on" relocation bracket money can buy, so keep us in mind for that item. Please remember, one-size doesn't fit all, so please choose a version that features multiple LCA mounting points.
Adjustable Upper Control Arm, to dial in the pinion-angle....which is also affected by lowering. This is a very subjective topic, but I always recommend adjusting the pinion angle after lowering the car. The factory has a pre-determined pinion angle for a reason.
Are you planning on running aftermarket struts and shocks?
I could go on for days, but those are the basics. If you have more questions, please give me a call and we can discuss.
Happy Modding!