Hunt around for a 1/2 barrel keg (15.5 gallons) if you either have, or have access to a couple of basic power tools. I'm talking about a decent/good handheld drill (something you can use at lower speeds, like 500rpm and less) and a right angle grinder. With just those, you can convert a keg into a keggle without spending a lot of money. Of course, that depends on the keg cost. For a basic keggle, all you'll need is to cut the top hole (as wide as possible) and then install a ball valve and dip tube. The
dip tube and
3 piece ball valve (all stainless) require just some
Teflon tape,
washer, and some
o-rings (1/2" NPT) or
flat silicone gasket, to be complete. Should only add about $45 (plus shipping) to the keg (or even pot) cost. Once you have that setup, you can use the kettle/keggle to brew. You can add more things later, like a sight tube/glass assembly. I've removed (or not even installed from the start) thermometers from my keggles. That includes the mash tun keggle I'm using. I've gone with other methods that don't add more through the keg wall. Basically, the thermometer in the keg mash tun interfered with mixing the grain more than it was worth.
So, if you can get the base keg at a good rate, and either have the above mentioned tools (or know someone with them) I'd make a keggle. If you only have (or have access to) a drill, then look to convert a pot. In that case, which you go with depends on your budget level. Aluminum will be cheaper to purchase, and easier to drill. Only downside is the need to condition it before you brew in it the first time. Stainless can cost more, and can be harder to drill, but won't require any conditioning before you use it.
BTW, unless you have a stove that puts out a LOT of heat, also look to get a propane burner.
