For stove top brewing, including stovetop ag I've managed for years with a 5 gallon pot I got at family dollar for 10 bucks. It's stainless even. My stove can handle a 3.5 gallon preboil volume, which I then boil down to 2.5 gallons.
A lot of my experimental batches I do are 2.5 gallon batches, but I have also posted on here how I do 5 gallon batches of all grain with the same pre-boil volume above, but topping off in the fermenter to a full 5 gallons.
You can read a detailed explanation of how to do it, here