Before you lower your car you should determine what outcome you wish to obtain. Some want simply a lowered vehicle with the same driving characteristics that they had before. Some want in created performance and better looks. When you increase your spring rate to make your car handle better it will ride more rough, how rough depends on the type and rate of spring you buy.
Linear springs are those that maintain the same rate all through out their compression. They never change how much their rate is compressed or not compressed. These are cheaper. You can tell a Linear spring generally by looking at the coils that make it up. They are all evenly placed apart (minus the very top and very bottom coil that may have to alter its shape to fit different cars).
Progressive springs have a changing rate. They have a softer rate under normal driving conditions and as you push the car to compress the spring more the rate goes up to a higher rate. This gives both ride comfort and handling improvement due to this nature. You can tell a progressive spring by its coils that will get closer together as they go up or down.
Generally progressive springs are better, especially for handling improvements, as you can have a much stiffer spring without the constant jarring of just normal driving getting to you.
A good spring rate for a street driven mustang is between 600-700lbs front and 300-400lbs rear. (If an independent rear end the rear rate will be higher).
The cost of the springs should be around 250-300 dollars.
Other items you may or may need to perform this installation:
Your stock struts/shocks are ok but if you go much lower than 1.25 inches you will wear them out much faster. For the best handling you need shocks/struts that are valved to go well with what ever spring rate you are using. This step is important for handling improvements. A good set will run you about 200-300 dollars front and back (Tokico mach 1 spec) up to 600-1000 a set depending.
Caster/Camber plates will be needed potentially at any lower height than stock. Some say more than an inch of a drop you need them. It all depends really as some cars need them and some do not. They help you get what ever alignment specification you want though, as the stock alignment may not be the best for your car for tire wear or handling. The price is generally around 200 dollars. Maximum motorsports seems to make some of the best and the strongest with the most adjustment on the market but tons of people make them.
Isolators, these are just rubber rings that go on the ends of your springs to keep the metal on metal contact from happening and causing noise. I would reuse the stock ones as aftermarket ones are generally thicker and raise your car when you want a drop to begin with.
The best drop will be between 1.25 inches and 1.75 inches. This drop gives you about the most you can have and still daily drive the car without to many issues. You will still have to take it easy getting in and out of places and going over speed bumps. Some want lower some dont. Most everyone that only goes with an inch drop regrets it as you can barly tell its there. I think the best drop gives you two inches of gap at the tire to fender for the front and a touch more than that in the rear. Your stock height from the ground to the top edge of the fender will be around 28.00-28.25. Mine currently is 26.0 and I have no issues driving around (well nearly none).
Brands to check out:
H&R Race Springs(probably some of the best for handling) at maximummotorsports.com 260.00~ (700+ rate front)
Ford C springs (budget linear springs but good bang for the buck) 140.00~ (600 rate front/300 rear)
Eibach Prokit (no idea on rates, seem to be softer for better ride) 200.00~
Installation: Its very easy and straight forward. Here is a link I wrote about it, if you have any questions fire away!
Installing Mustang Lowering Springs
All in all this is a VERY nice modification that will amazingly change the feel and look of your car when done right. Wheels top this off and change the car very much.