As you said, the kidneys are great about excreting potassium. If you have functioning kidneys it will be hard to achieve via oral potassium. Potassium is a known irritant to the stomach, so the amount you would need to take to get your levels high enough to induce cardiac arrest would be well beyond what would make you throw up. If you have the medical knowledge and access to IV potassium, it's one of the most painful IV medications available to the point that pushing a lethal dose may not work due to backing out from the pain. I've thought it over so many times trying to think of a way to do it and unfortunately it doesn't seem like a reliable method unless you have kidney failure.