A great bang for the buck would be the SVT front brakes. 300mm rotors & bigger calipers, that bolt right on. You will need 16'' wheels.Some 15'' might work. Your stock brake lines will work, but I install new ones or upgrade to Stainless steel lines. Your cheapest route for the rear is to keep the drums & get new shoes & turn the drums or replace them w/ new ones. Yes you can convert over to disc. The '05-'07 ST's have 10'' disc which are more readiy available, then the svt's 11'' ones. The stock master cylinder will work great. Hit up the B&S section. There are a few sticky's on this subject at the top of this section.
Introduce Mr. Torsen to the mix
Bigger brakes then you- D23 crew- D-Tech- Sedan Revolution= #131