STi 5 spd is rarer in the us than the 6 SPD.
If your gearbox works you don't need a gearbox. If it doesn't you can buy three WRX 5 SPD boxes or a single complete STi swap. I'd buy WRX boxes one at a time because by the time you go through them all the chassi may be don anyway. I'd not throw $6K in driveline costs at a car worth $6k. Hope this makes sense.