coil over kit basically impoves the efficency of the spring location. you can run a lighter spring rate and still have the same mechanical advantage. from what I have read they say that a 200lb spring on a coilover setup is the equivenent of like a 800lb spring in the stock location so you can fine tune your spring rate easier. you can also have adjustable ride hieght with coilover.
for example coilover they will suggest like 250-350lb rating. were if you tried to run a 1000lb spring in the stock location you would have no suspention basically as the springs would be just to stiff.
the dissadvantages are cost and a poorly designed kit will be noisy. If you search you will find many complaints of increased noise with coilovers.
I would proably only try griggs or MM on coilovers but I have heard praise and complaint of both.
bumpsteer correction cam be done is some small exent with offset rack bushings sometimes but mainly you would get a bumpsteer kit which are tire rod ends that alow you to raise of lower the angle of the tie rod to match the setup of you praticular car. you arms and tie rods need to stay in relation to each other. so this alows you to ajdust that.
You should read this about coil-overs and Mustang suspensions in general; http://www.miracerros.com/mustang/t_suspension2.htm#Springs A good kit would be from Maximum Motorsports or Griggs. If you have the $$$ go for it. Also you can adjust ride height with coil-overs. Just stay away from cheap kits like from Granetelli.