Kit: I'm pretty sure you can get it cheaper if you order one part at a time...but I could be wrong.
Ball joints: you don't necessarily have to get Steeda ones. I'm sure there are cheaper options that are just as effective.
Bumpsteer: Some people hate it, some people love it. I personally don't like them, but if you are planning to lower the car I would recommend it. Theres hardly any use for them at stock height.
Bushings: Yes, I would replace all the bushings. If your suspension is shot, then more than likely the bushings are shot as well. Considering they are fairly cheap and you are going to have to replace them at some point anyway, I say go for it.
Do you plan on putting on some thicker sway bars or just leave the stock ones?
1998 GT - CAI, trickflow 75mm intake, 3.73 gears, O/R x-pipe, flowmasters, short throw shifter, 24lb injectors. 183k miles
1995 GT vert - 1 of 36 5.0 Appx. 400,000~ factory motor miles (totaled, in garage)