The most critical need for an electric brew kettle aside from the obvious electrical safety stuff is to be able to throttle your 240 volt heating element during the critical period between hot break and boil. The PWM kit that Stilldragon sells works well, make sure it is wired correctly when you build it. I use a 5500 watt element and it can keep up with a propane system, at a fraction of the energy cost. Your big expenses are going to be wire (#10 for a 30 amp circuit), if you have to run it a significant distance from your main main panel, breakers, and outlets. That kind of stuff goes up steeply in price once you get above your 20 amp 120 volt grade electrical hardware.
Another consideration is if you are doing a keggle is to have a good way of making a clean and proper sized hole for the heating element. My dad loaned me a set of chassis punches he got from Harbor Freight, and I quickly proceeded to destroy them. I ended up buying a used set of Greenlee punches off of Ebay for about $50 or so, and they will be available for future projects.