Depends on the batch you want to brew.
Keggles are 15.5 gallons - 70 quarts. More than enough for most. You can get aluminum kettles up to 40 gal - 160 qts (and probably bigger). You can get one here for $100
+shipping. May be overkill, but a lot of people spend a lot more than that for a small SS pot. Some kegs(commercial/sanke) are also aluminum.
If you're going to buy a keggle, make sure it's stainless
Also, most stock pots are thicker than a keg. More expensive pots will have a pocketed bottom to allow heat diffusion. If you get a keggle or a single layer stock pot, try to find a piece of flat steel to use to distribute the heat and prevent scorching.
If you are going to use a keg as a boiler, see potential risk