[wawazat??]

Transmission, front differential ratio, rear differential ratio,
clutch release type (cable, push hydraulic, or pull hydraulic), pedal assembly to match the clutch release type, clutch type and brand, flywheel type, drive shaft modifications.....

Transmission costs range from $200 (older Legacy cable type) to $5000 - $10,000 for a 6 speed (used vs. new). Depending upon the transmission you choose, you'll need the pedals/linkages/cable or hydraulics. I've just bought a set of pedals from a new WRX with linkages for $150. Clutches can be OEM, performance aftermarket, basic aftermarket. Cost here is $100 to $500. Flywheels are the same thing depending upon type, weight, material, OEM vs aftermarket.

Also depending upon transmission you choose you may or may not need a rear differential depending upon the front differential ratio. Available rear differential ratios are 3.54:1 VLSD (SVX and new USDM WRX), 3.90:1 VLSD and open(Legacy Turbo, certain Outback models), 4.11:1 open(Legacy, Forrester, Impreza) and 4.44:1 open(Impreza, Forrester). I've spent a lot of time at my dealership making copies and tracking down info. Again costs vary from dirt cheap to $750 for a complete new rear diff. I just bought a Legacy turbo 3.90 LS rear diff for $200. If you were to find a new WRX transmission, you can use your existing rear diff as the USDM spec WRX transmissions use a gear ratio multiplier on the output to the rear end to match the 3.90 front ratio to the 3.54 used in both the WRX and the SVX. This makes for an easier swap, but WRX transmissions are the most pricey of the 5 speeds ($850 -$1200).

Then you have mounts, driveshaft modifications, wiring, shifter and modifications to it.... a lot to think about.

That said, I'm doing mine this winter. Most likely with a Legacy Turbo trans, but possibly with a WRX trans (I'll know later today). I also have a complete older Legacy trans/rear diff/pedal set using the 4.11:1 ratios. Choices, choices, choices....

Todd