I asked Nathan at Brewha specifically for the material used for the inner vessel construction, here's his reply:
Hi Ken,
Thank you for your inquiry!
The 3-in-1 is made with 1.5mm stainless steel. It is durable. I don't know of any homebrew equipment that uses thicker material (though it may be out there) and I am aware of many companies that use 0.8mm and 1.2mm. I list the pressure at 3psi for vessel and 5psi for jacket as this is a very safe margin for operation while allowing a brewer to do everything that is necessary for the brewing process.
To build the vessel for 80psi (e.g. municipal water) would require 1/4" steel, which would increase the cost dramatically as there would be additional material, transportation (it would be much heavier) and certification expense (anything above 15psi needs to be certified as a pressure vessel).
Does that address your concern?
1.5 mm stainless steel throughout is 16 ga. Seems like a reasonable gauge. In fact, it's industry standard for beer kegs.
For example, here's a link to another company that makes kettles and they are quite proud that theirs are made of 1.2mm steel.
http://spikebrewing.com/pages/faqs
The Brewha seems to be well engineered and well built. This thread did help to make me do a lot more research into this system, but after analyzing what I've found, I still have the confidence to give Brewha my $5000 as I believe it to be well designed and well built. Maybe not perfect, but as good or better than anything else out there.
I've looked at the Sabco, but I don't have the floor space and I don't want gas. I've looked at the Blichmann Breweasy, but I'm not as impressed with the design or the execution, plus there's just more pieces to move around.
I'm prepared to pay the cost for a superior system. If once I start using it I find it less than satisfying, I will be sure to report my findings.
I will let the forum know in any case.