Safety is a prime concern when working with lines voltages and high powers like this. So, take the advice of the previous poster and put a high limit switch in as a final resort in case of faults. Follow the typical practices that you see in other equipment, where all lines voltage connections are made with insulated connectors or are potted so that anyone reaching into the circuitry can't touch a live high voltage conductor.
Also consider that if your device burns your house down, the insurance company probably won't cover it.
You should also beware that you are going to need a great deal of line regulation before your low voltage circuit. When your element switches on and off, the line voltage is going to jump up and down a significant amount due to changes in voltage drop. So you need to put in more line regulation and filtering than you normally would with typical low voltage circuits. Perhaps you should consider a high performance linear regulator and also consider cascading two linear regulators, the first outputting a higher voltage than the second one requires, of course.
One reason that a lot of simple appliances use non-electronic temperature regulating controls, like bi-metal strip switches and the like, is that they are simpler and aren't sensitive to the tough conditions involved and probably more importantly, cheaper than the solid state alternatives. Crude but effective. Electromechanical controls are not sensitive to static, are slow so they don't respond to glitches, and don't have RFI problems. And did I mention that they are simple? For example, a percolator that I opened up recently had only two controls, one being a temperature regulator device that used a bi-metallic switch and the other being a timer device that was simply a variety of temperature sensitive resistor. Now, that's simplicity.