Hah yeah man, if I used wood I would need to paint it so it does not rot. The humidity in there is going to be very high, and if I painted it I would be worried about what the paint off gases when the green house gets very hot inside.
Once I get more power out in the workshop I plan on getting a basic welding rig and playing around. Should be interesting.
Oh, okay. I was thinking you could use something like cedar that wouldn't need to be be painted, but the metal is probably a lot more solid anyway.