If you go with the stock ecu, you will have to do some wiring on the body harness connector. You will hopefully be able to open up the ecu to mate the wires up. As for a stand alone, you can't exactly buy a stand alone that has precut wires for 3SGTE in a celica. So there will be plenty of wiring if you go that route.

Just out of curiosity, is the mega squirt a piggy back or an actually stand alone that replaces the factory ecu? For that price I would think it was a piggy back just like an S-AFC or S-ITC.