TJ-

First of all, if you do this, I'm moving to your neighborhood. Vapor barrier for sure, either some 6 mil plastic sheeting or a small roll of Tyvek, whichever is cheaper based on how much you'll need. If you wanted to go full out, you could line the walls with SC or try to find a SC veneered plywood. Are the ceilings 8'? If so, it looks like 4 4x8 sheets of plywood would take care of the walls. I think if you didn't line the walls it would be fine, as big as that is, SC shelving would be awesome. Either way, it would probably smell like heaven.

As for expansion, if you let what you are going to line the walls with/make shelves out of rest in the humidified space for a few days it should acclimate and be fine to just butt the ends as long as the room itself wouldn't have big temp swings on a regular basis.

As for materials, when I turned an old gun cabinet into a humidor (and then did another for a friend) I used Bristol Valley Hardwoods Bristol Valley Hardwoods – Hardwoods & Speciality Wood Products They will even (or did several years ago) cut to size before they ship it.

If you do this, you must post pics. Must. Post. Pics.