To the above reply; there are a lot of app 13.5 gallon kegs out there, some from Europe like France the most i've seen. This will screw up a brew system when you have all your other normal 15.5 gallon kegs operating at their maximum volume. That pony can be used for your HLT depending on the volumes you want to brew.
One European company makes pots not kegs that have metal turned finish sides that come in different sizes. One size I see a lot is the 50 or 50,000cc's in volume. This is to the brim so no boil or foam head space above besides only holds 13.21 gallons in volume and not cheap. Your paying for this pretty in finish "heavy 18 gauge thickness" material pot. How can the words "heavy 18 gauge thickness" change the pots thickness from the words 18 gauge thickness? Thes pots will limit your volume unless you purchase their larger volume pots. These pots are larger in diameter also exposing more surface area on the boil for more boil off vs a 12" opening in a keggle. Heck go to a 10" opening window in a keggle. Also look into how much heat can be applied to these thin bottomed pots if propane or NG heating is used before possibly causing any scorching of the wort.
I would rather have a thick walled keg converted to a keggle than a "thick 18 gauge pot" besides the money saved on keggles have better heat retention due to the mass alone. Talking 33 pounds for a 15.5 keg.
A balanced system is (what I call it) when all the container volumes are reached at their maximum together, by this the HLT, MLT, Boil keggle and fermenter all full without no one keggle holding you back forcing you to reduce the brew batch volume on your brew day. Sorry if I ran off topic here I just wanted to get a point across with adding the whole picture on building a brew system from scratch. To not have you waste money time and your labor. Start out right once is a hell of a lot cheaper than a live and learn process. Especially if you have to explain it to the War Department about your screw up. End of rant as I have been called only a "visitor" on this forun by one HBT member. CJ............