I have disc wheels and have used them fully loaded for touring. I don't think you'll have strength issues assuming they are decent rims and have a decent build. FYI, I have a 36 spoke count, which I'd recommend for touring given a choice of 32 or 36 hole rims.
Having said that, aren't XT disc hubs more expensive than non-disc hubs? And if you aren't going to discs, why bother with the added expense of disc hubs? If you have the time, shop around and get the non-disc XT hub -or at least buy one yourself and then give it to the LBS to build up (after all, you did say they didn't have one).
Better still, learn how to build your own wheelset and buy the components for less online and build your own.