Do you already have the chassis? Have you looked into starting with a Legacy RS (EJ22T block, factory turbo, etc)? Is this an endeavor you're looking to perform yourself or would you be bringing it to someone who has more experience and fabrication capabilities?

If you're dead set doing a swap, the main items you'd want to focus on are:
EJ257 block, recommend CP pistons and Carillo rods built by a professional
Turbo front crossmember
Rotated Turbo, whether it be a modified Perrin, Ultimate Racing or custom made with front mount intercooler
Walbro 255 fuel pump
STI 6-speed conversion with R180 rear end
Fuel injectors
Wiring service (probably not something you would want to engage in unless you have a lot of wiring experience)

Although the hard points for things like the front and rear subframes are largely the same from 89-present, there are quite a few sheetmetal differences that will prohibit you from using an off the shelf downpipe, FMIC setup or even TMIC.