Those Xtracycles look pretty neat!

I have the 26" wheels that come with the full Surly LHT build from 2010. 36-14g DT Swiss spokes (I don't remember if they're butted), Alex Adventurer rims, Shimano Deore XT hubs. I'm pretty sure they're handbuilt and am 100% sure they're very strong. I load my bike up for commuting, couch-surfing around friends' places, getting lots of groceries, etc. and I, myself, usually weigh between 205 and 220 pounds . I'll go over small bumps while loaded up at a decent speed and a 1.75" tire and they stay true. I don't baby them and I don't abuse them . I never worry about them but don't try plowing over curbs at speed or anything else considered very abusive. :-p I do carry lots of weight, though. More than most commuters and such, because I want to be prepared for when I start touring soon. I've also taken it onto packed dirt paths with some roots coming up here and there unloaded without issue.

The longer tail design of the Xtracycles redistributes the operator's weight more to the front wheel. This puts more stress on the front wheel and less on the rear than most bike frame geometries when unloaded. Then you will be adding a cargo and passenger load to the rear. Both of your wheels need to be stronger than average. I think 36-spoke wheels both on front and back will be good for the way your bike will distribute the weight. Also, I'm on the same page with some of the other posters about not getting extremely beefy rims. I don't think you'll need them since you won't have two adults riding the bicycle. The 40-spoke and 48-spoke wheels are usually for tandems. :-p