You've gotten some really good tips, here!
Concerning cooking just one pork butte.... a lot of guys like their wsm's with no water in the pan, and my point isn't to argue the merits either way. However, just keep in mind that the newer WSM is pretty big, so I'd make sure I always kept enough meat in it for the sake of moisture if you took the "no water in da pan" route. (Actually, it goes without saying, but I'd suggest first learning how to cook with water in the pan before ever trying dry if you're cooking at traditional low bbq temps.)
You've probably read about how the factory gauge should read thirty degrees or so lower than the top grate cooking temp, so keep that in mind and don't think you need to be in the middle of that "smoking range" if trying to cook in the traditional 200-250 range. I like my factory gauge a tad over 200, but rely on the Maverick to target 235 at the grate.
Assuming a traditional low and slow cook, smoke overnight to be on the safe side, and have a decently insulated cooler that you can pre-heat with hot water to hold the wrapped butt in for up to five hours or so if it's done early. (I kept a few for six hours once and they were still plenty hot, and turned out super moist.) Just fill the cooler with crumpled newspaper around the wrapped pork butt.
Good luck!
Dave
UDS, wsm, wots, char-griller