Back in the day, I had a totally DIY E-HERMS before most of the "modern" kits were available. I strongly suggest avoiding the 15 amp one. High Gravity Brewing has some simple pre-built control panels that can handle a 5500 watt element. The prebuilt panels are widely available now, and you don't need to spend 2 grand, but probably at least 400 bucks for a basic one. The one I built years ago was based on Kal Walner's book, but scaled down for one PID and some switches. If you use good quality parts, you will still spend a fair chunk of change on it. A PID is great, but not absolutely necessary. A simple stove-type infinity switch with a contactor is also a popular choice.