there are plenty of people that the stock tranny has held up for, but youll need a different clutch.
the EJ20G swap should be easier to wire up since its OBDI while the USDM EJ205 is OBDII.
if your not looking to make HUGE power (not that its not capable of it but $$$), then the EJ20G is fine, there just arent as many "bolt on" parts that work for it as the newer engines.
the wiring would not be that hard since its an old engine, pretty straight forward adn youd have all the wiring you need with the engine.
the EJ20G is a better motor than the EJ205 also, it comes with a 16G turbo, with other STI parts that a standard WRX doesnt have, just less of an aftermarket for it, plus the mileage.