Edit: I'd go with the Mach set up and the "F" lowering springs. The Mach kit comes with sway bars, shocks/struts for around $299 (Bullitt kit has springs too and is $399). Since you want to lower it so much, you won't need the Bullitt springs.
The Ford "F" lowering springs are about $210.
You could probably leave your stock shocks/struts on for awhile and get the Ford "F" lowering springs and c/c plates for right at $300. That will get you the drop, but you stock shocks/struts will wear out quickly.