I looked extensively at using round bar or pipe for a brew stand. Round bar is overkill as far as structural needs go but it will work if its cost effective for you. Round pipe would work, I would recommend Sch 40 or 80. Round tubing can be used I suppose but why risk it...you are talking about potentially supporting 300 lbs of boiling liquid...you dont want that to collapse on you.
The trick is assembling it. There are MYRIAD pipe fittings what can be used. But, t and here is the rub, they are either made of aluminum which is NOT structurally sound at the temps this thing will reach, or galvanized or iron...which why bother using SS bar if the fittings are going to burn zinc/rust.
I really wanted to make it work as I have a good resource for Sch 40 SS pipe on the cheap but the fittings made it untenable.
But if you can find a solution that I overlooked it would be a really unique, awesome stand.