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The ESP8266 did not need a level shifter for 5V inputs from the serial port on the MEGA
The only problem was that we needed a 3.3V FTDI for the remote ESP8266 and the common FTDI would not support enough power for the ESP to run all the time. I see that there is a new SparkFun Beefy 3 - FTDI Basic Breakout (DEV-13746) that should solve this. Otherwise you need a battery or other power for the remote ESPThe Arduino MEGA 3.3V regulated output ran the local ESP just fine.
3D Printed Stepper Motor V1.1