One of the things I like most about the Mustang is how cheap aftermarket parts are - a couple of years ago we installed a tubular k-member, a-arms, and coilover conversion on my wife's car.  All of it came in under $700, was easy to do, and makes a huge difference to the handling of the car. 

The thing that put me off so long was that I couldn't figure out the coilover bit because of the modified MacPherson struts - since you're supposed to be able to use your existing struts, I just wasn't sure how that would work.

Basically the new lower coil perch is a sleeve, cut close to the perfect diameter, with a locating notch to align it where the tabs come out to bolt to the back of the spindle.  Everything is really solid, and it doesn't make any weird noises.  Incredibly happy with this - but I'll recommend that you upgrade your struts before going this route, because obviously 10 year old Pepboys specials aren't going to thank you .