Jesus Christ on a cracker THE ENGINE DOES NOT NEED TO BE REBUILT.
It probably does not even need new head gaskets.
Drive the car around some, or at least let it run up to operating temperature, then rev the engine a few times and see if anymore smoke comes out the tailpipes, does it have leaks anywhere, does it run funny? If there is nothing more to be seen then it was a slow leak while the motor was sitting that purged itself on startup, and not worth concerning yourself with so long as you watch coolant level and watch for future signs of trouble.