* i have occasionally ordered a fiber drum which had an 8 mill food grade poly liner. They are readily available and used for rice enrichment, snack foods, some chemicals. If a little hunting did not turn up a gusseted bag you could accomplish the same thing with heavy gauge poly roll stock lining the tank. To look pretty corners could be taped to form gussets.
On occasion I had to produce 1000 lb bulk bags of product, larger sizes for 42 by 48 pallets are readily available in food plants.
* If i had a new stainless tank I would wash with the strongest detergent available to remove machine oil (well with driers we would burn the oils at 350 to 400F for a few hours). After degreasing I would take an alkali cleaner followed by an acid rinse. (Do you have a meat plant near you?) (milk or ice cream?) (university pilot plant?) (Klenzade distributor). If there was a pesticide in the metal I would wash more and possibly soak with an acetone or alcohol after degreasing.
* brass is in some valves which should not be used with wine. A good setup usually has a stainless NPT fitting welded in so valves or pipe can be changed out. Some have tri clamp Fittings. Brazing ports or fittings is to be avoided again for copper content. Silver solder is OK. Gas blanketed welding is standard (?TIG) as a food surface. Since your dad did the work what did he use?
* winemaker conference had a vendor in 2019 who was selling a pallet size metalized film, low oxygen transmission spouted bag liner. pallet size (plus others?). There should be a vendor list on the web for that meeting. Industry wise they are found in bulk tomatoes, tomato paste, aseptic pack fruit, aseptic pack juice, concentrates,,,,, so again what kind of food plants are nearby?
* used equipment dealers might do heavy duty cleaning as a standard process when they take possession (Rabin Brothers) and might be worth talking to/ have small cups of chemical.
Nice to have a dad to help on the toys!
,,,, edit after sleeping on this, heavyweight clear plastic poly liners are fairly common for lining 55 gallon drums for garbage at football, baseball, nascar, concerts. You don’t want a hundred count case but your high school and A to Z rental may have them on the shelf. They will not say food grade, but the resins for blowing films are clean. Polyethylene has a measurable oxygen transmission rate so this isn’t long term storage grade material.