Looks good. I assume you will use a spindle that does not already have a speed controller built in. Are you going to try to control the fan PWM in gcode with your postprocessor or are you just going to use the LCD knob to manually control it. Either way is a great idea. That SSR module might need a heat sink. You should probably measure the temperature on the back with your spindle on to make sure.
Let us know how it works out. FYI here is a teardown of that type of SSR https://www.youtube.com/watch?v=DxEhxjvifyY.