What should be bought 1st? At what point are CC plates manditory? My car sits higher in the front than the back...Do off the shelf lowering spring address this?
I'd recommend you look at what the car needs to do for you before you decide how much to lower it.
Maintaining that OE ride is easiest done with the Ford Racing spring sets out there, with about 3/4" drop the most before you start sacrificing ride quality.
If ride quality is less important than handling, consider buying your springs from a company that sells the whole ball of wax. Maximum Motorsports :: The Leader In Mustang Performance Suspension
, Steeda Autosports | Everything Mustang Performance
, or many other sources offer everything you need to transform that car to a real road track handling machine.
If you want the car to drop more than the 1.5 inches mentioned by lxarlo, it does indeed get more complicated. Steeda suggests adding their torque steer kit at that point, and I agree with them. CC plates are a good idea because they can help correct the changed geometry created by the drop in ride height, and allow adjustments on the track, which can greatly aid handling in those conditions (though you want to keep those adjustments more neutral for the street).
There ARE other alternatives for lowering your car. The most expensive is Air Ride, which will allow you to drop it at shows down to the ground - raise it high for street driving over speed bums and such - and lower it to match the needs of a road course. The other, less expensive (though considerably more expensive than a simple spring swap) alternative is coil-overs, which replace both your existing shocks and struts and springs. These come 2 ways, with single and double adjustable versions (the double adjustables cost considerably more).
A lot depends on where you are going with your plans for the car.
For a simple street car daily driver, my advice would be to drop it 3/4" and compromise...
But for that mean look, I know that more is more...
Its the choice you ahve to make. I went toward the edge of the road course route with my 2002 and a bunch of Steeda parts. She rides a bit rough, but handles like she's on rails compared to where the car started.