It looks like all the parts are stainless - tank, burn chamber, and hx tube. The tank is magnetic, so I'm assuming the less expensive 400 series stainless, or work hardened 304.
We'll see how far I get trying to roll a flange with my Harbor Freight tube beader set. It says 18ga for mild steel, but I added a 1/2" thick strap right behind the roller to beef it up. I might have to get some steel, and turn some mandrels.
JMESanitary.com has butt weld stainless 1.5" tri clamp fittings for under $10ea, so I'll get some this week. Have to get valves (thinking 1.5" butterfly for the trub dump) and come up with a racking arm setup too, but that could always be down the road.
For the back gas purge, I'll be going cheap with a Y after the regulator and some aluminum foil. The solar flux paste isn't for use with food, so back purge it is.
The welds inside look OK. I may try to polish them out. I'll see if I can get some Nitric Acid and HF to pickle the tank after I'm done welding.
My parents neighborhood all had the Lennox CompleteHeat systems in their new construction around 2001. I'd say 75% have been replaced by now, but ya can't beat a free stainless tank!
We gave the board & blower assy to a neighbor to keep as spares.
Thanks for the comments. Fun project!