he's referring to the waterproofing when he says 50 year, that has nothing to do with the ply tank's structural integrity or any mention of a warranty for workmanship and design. Also we still don't have any idea of what he uses, previous builds to compare quality, whether or not it's a complete kit with stand/glass and etc. There's a lot more homework you need to be doing here, remember that 450 gallons coming at you and a dead aro is exactly what you want to avoid. Any homework now is well worth avoiding that mess.
If you're only communicating via email right now I'd say give him a call to detail what you want and then have him send you a formal quote. If he can't provide that move on, the last thing you want is a non warrantied ply tank built by "plan it as I go" jimbob.
