What most dont realize is most aftermarket springs have close to the same lb rating that the factory ones do. To maximize your suspension setup without going all out springs can be left stock. Lowering your mustang can become expensive quick by the time you add a bump steer kit and caster camber plates to compinsate for the drop.
Next thing you have to consider is that tires make the single largest impact on any cars performance capability and use when it comes to traction. I recommend picking out the best tire you can for your aplication before starting on suspension setup.
First place to start modifying suspension and maximize your investment wether your going to do corner carving or drag racing or just need some extra traction and launch ability for your street car is firming up the rear. Upper and Lower control arms, adjustable struts, sway bar and sub frame connectors. Also if your running a fox I sugest the very first thing you do is have battle boxs installed so your rear end doesnt deside to just rip itself out of your car one day.
Picking suspension peices isnt as easy as you would think. Price per part aside you now have to decide what am I really going to be using this vehicle for? Will I run it at the track sometimes or only? Am I building this on a budget or am I trying to set records? Do I care about ride quality because its my daily driver? Will I be running a power adder? Street tires?
If you can tell me what you will be using your car for and tell me your goals I cant help you with a parts list that will effectivly get your car to your goal without wasting your hard earned money.