I'm about to start down this same road. The setup you showed at the beginning is exactly what I started with except I just got ahold of an old keg someone had laying around and cut the top off of it. I have a large order about to go in at brewhardware.com for just about everything I need for the build minus the electrical and control parts.
I was going to do the solder on triclover ports for the heating elements so they could be removed for cleaning or capped off in case for some reason I needed to use my old propane burner.
Do you think you could get away with a cheaper heating element in your HLT since scortching is less likely to be an issue there? I've considered just using the wavy 5500W camco element on the kettle and using something I can get at the hardware store on the HLT.
After having ran with this system for a few years now, is there anything you would have added or changed at the beginning to make things easier or better?