Wow husky, I have only just seen this. Awesome work on the kettle! Absolutely magic!
I have a very large SS conical (~1000 litres) that I will have to talk to you about modifying and then we would have a matching HLT.
The three issues last time were ramping 500-600 litres of near frozen mash, only have two 10 amp outlets for the HEXs and relying on a timer (that didn't work) to start heating at 6am (rather than the time we noticed it didn't start - 9am). The lessons were, don't do a cold mash, have more than two 10 amp outlets, and KISS. As Nath mentioned, Mr Cockup hasn't missed a swap in a long time.
Ideally we want to mash in at 60 for a 55 rest. In July, we will be doing this with an instant hot water service. With regards to outlets, you really want at least 4x 10 amp outlets, ideally on two separate circuits. Keeping things nice a simple helps too, in the past we have used STC-1000s for running the HEXs, my preference is to upgrade these to PIDs with SSDs but I would need someone to design this for me, as I have no idea (similar to Wiggman's). PM me if you are game.