Going around corners, you will never truly have a good setup until you overhaul the stock 4 link rear to a 3 link, 5 link or the IRS. And the IRS comes with its own host of issues that need ironed out before install.
If its just a street car that will see limited handling or never a real road course...
Good lowering kit
MM or Steeda LCAs
Good set of wheels/tires
That's pretty much all you need. After all this is when you might look into converting the rear because nothing you do to the 4 link can really make it hold the rear in place around a corner. The stock K member and A arms are actually ok handling wise and can wait until the rear is sorted before going nuts with aftermarket tubular stuff.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.