At $224, the 18.5 WSM is a great deal. Buy it!!!
I've loaded my ring for a pork shoulder, filled the water pan, and have gone 15 hours without having to reload. After the smoke, I opened the vents (water was gone in the pan) and it shot up to 300+* and stayed for another couple hours. Cleanup is not a big deal, just line your pan with foil, and instead of water, you can use a clay saucer, sand, etc., but I prefer water. It takes a few more minutes of cleanup, big deal.
A full packer brisket is going to be tough at first on an 18.5. You will have to tuck the ends under the handles of the grate, if it's even that short. The 22.5 WSM will handle one no problem, but that's not on sale. But I think for another $100, you can get a 22.5 WSM from Amazon, which is a great deal too.
The first couple smokes, it will burn hot until it gets some seasoning and you might have to fiddle with the lower vents more. But once the seasoning is there, it's like setting cruise control.