Given a cylinder of 12" diameter and 36" tall inside dimensions would be about 17.5 gallons. If those would be scrapped - let me know as I would be interested in picking some up.
Being a professional welder I can tell you that "sanitary welds" are not at all just a ploy. However, that being said, it's not incredibly difficult to do. Sanitary welds simply means there are no cracks or pinholes where bacteria or yeast can settle into and avoid the cleaning process. That usually means nice, clean, shiny welds and usually with a good bit of practice anyone with a steady hand and a good eye can easily accomplish this (especially with the TIG process).
Also, as far as the lid is concerned, equal pressure around the seam to prevent "waffling" is all you need so reinforcing the lid and/or collar should fix and leakage issues and would essentially give you the same or better pressure tolerance of a corny keg. That of course is subjective of the hopper's wall thickness and integrity.
All in all, this is an impressive project and I will DEFINITELY be trying this myself!
Oh and p.s. Stainless paste Flux is a hit or miss, I've had it work great and seen it sugar just as bad as if it never had anything on it at all...welding temperature probably has a lot to do with it but I haven't played with it enough in my career to truly get all the nuances and bugs worked out. Almost always back gassing is the preferred way. Since it is a conical hopper you could use something like a pot lid with a silicone ring around it and find a way to make a seal inside to prevent wasting your argon less is more