Theoretically, you could set up an all manual system of switches, but you would most likely drive your self crazy trying to keep all your temperatures in check. I couldn't imagine having to flip switches all day, I don't think I'd have time to brew.
How do you plan to mash? Is your MLT just going to be a direct gas fired kettle that you stir manually? If so, I'm not sure I would worry about an element in your HLT since it's a plastic cooler. People sometimes struggle keeping the element from leaking in a plastic cooler. When I had a plastic HLT, it would keep my sparge water at the correct temperature without needing to be heated. Of course I was careful not to add the water too soon, and I calculated a few extra degrees to compensate for the HLT not being pre-heated.