One thing I learned in my fish shed is that bare 2x4s will get moldy pretty quick, and it's a lot easier to paint the pieces of the stands before you put them together than trying to get around them after assembly. Otherwise it looks excellent! Definitely the way to go with lots of insulation and a single heater.
Looking good Jim. One question - why black for the walls? When I did mine I painted the walls stark white in an effort to increase the effective lighting in the room, and I'm glad I did, I think it really helps with visibility.