Yes, it is all we need to find salvation.
No, it is not enough if you want to establish a whole new religion that is totaly different then the examples given to us by the apostles, then in that case, one needs extra biblical or oral support to back up it's new doctrine.
If you think this is true, explain why God didn't rely on only the Gospel of John being written, but instead saw the need to establish a visible, teaching church with bishops, deacons, and prebyters?