I would suggest using metal studs or treated pine for the framing. The finish could be a number of things depending on how simple or exotic you want it to look :-) HD and Lowe's both sell corrugated plastic roofing panels in a number of colors, they actually look pretty good when installed as a wall finish too. There's also "Hardie Board" siding which is a cementitious product that looks like lapped wood siding once painted. Or you could use cementitious backer board over the framing and tile the whole enclosure.

Hey, tresw!
I knew you'd come through! I like the idea of the corrugated plastic roofing panels; I'll check it out over the weekend. I spoke w/ Libart as well; they do custom construct utility rooms so he'll work on the numbers...
Thanks, as always, for your input!
joanneswimsct