1. Think carefully about where you want the cable to plug in. Most people go for L or R side, depending on where the kettle is going to sit. I have seen angled plug-ins on the back - just make sure that the element will clear any valves, dip tubes etc.
2. Low is good, but make sure you have enough space below the hole on the inside and outside for the fittings. Kettles have higher bottoms on the inside then outside. I've seen a hole drilled too low, and the inside fitting would not fit. Some fittings use large electrical boxes on the outside.
3. I can wholeheartedly recommend the 5.kW ULD elements that mount to a 1.5TC fitting - easily removable for cleaning. I got mine from brewhardware.com. They have a weldless TC fitting to go with it.
Remember think about all aspects (sleep on it), measure 3 times and cut once.
Good luck!