I think you can just use the Arduino Mega as the whole sha-bang, I don't see that you need the complexity of adding a R.PI for the web interface.
That can be done very successfully in the Arduino.
For AC isolation, do some research on SSR, 'Solid State Relay' I use them for AC load control, look here Solid-state relay - Wikipedia
Also, for temperature control, think about implementing the concept of having a PID control loop.
Using PID will give you better control than simple limit setpoint control that will ramp up and then ramp down continuously.
PID needs to be understood, but is a very effective control strategy.
Again, I used it in my Mega, and it only consumes a handful of lines of code.
rockwallaby . . .