For less 100 bucks or less, you can get aluminum kettles at the restaurant supply houses every day of the week. I have a 30 gallon BK that hit my door for around 120 bucks. Don't waste your time with something that will cost more and never work. Here's a 35 gallon pot that I found in less than 30 seconds for $140. There are much better deals out there as well.
"Hard-side coolers: The exterior body of most Igloo hard-side coolers is made of high-density polyethylene, while the interior liner is made of FDA-grade polypropylene. The insulation is made of Ultratherm™ - a proprietary blend of polyurethane foam."
Polypropylene is good to 160 oC and is what my electric kettle is made of, the one that I bought from the hardware shop to boil water for tea.
HDPE is good to 130 oC. I have a 60 litre HDPE barrel that I have used to cook grain by steam injection at 95-100 oC for 2-3 hours with no ill effects. It is supposed to soften slightly at this temperature range but I have not noticed any warping or other problems.