Optocoupled MOSFET drive
A gate resistor is not necessary, the opto-coupler will turn on slow enough.

The MOSFET on the photodiode side is only necessary if your source can't sink enough current.  Since this coupler can be operated on less than 5mA, it is very likely you can operate without it.

I would lower the current through the diode.  Looking at the datasheet something between 1 - 5mA should suffice...so a 1KΩ to 5KΩ resistor could work.