I am not an electric guru as I am just beginning my electric build but from my research on this site, the most common used element for the boil kettle is the Camco Ultra Low Watt Density (ULWD). The power rating will be based on your requirements.
In the BK, once the boil is reached, the PID will not run 100% duty cycle. It'll vary based on the individual system but the norm I've seen would be between 60% and 85%. From my research, using a ULWD will not scorch the wort.
Last week, I installed a 30A GFCI breaker with a 4-wire outlet in my garage. I bought the 5500 watt Camco ULWD and today, I should be receiving parts for my controller. With any luck, I should be going within a few weeks. 'Till then, I can only speak from what I've read. Hopefully, someone with actual experience will chime in.