I think you definitely want a barrier between existing walls and new wood. Don't forget ceiling and floors your not trying to humidify the whole house. Now spanish cedar is great for a humidor but its expensive. Now spanish cedar is in the same family as mahogany but doubt that's any cheaper. Other options are cherry, walnut, maple, oak. IActive humification will have to be used but I would also suggest fans for airflow and maybe other media beads or kitty litter. The more you have in there the better to help hold the humidity.
For the electric I would add an outlet. Same outlets they use in bathroom will work in there. I would want a light on a switch and as I mentioned earlier led stripe lights on the bottom of the shelves to be able to see everything. Etched glass door and maybe consider a small built in in the center with drawers for singles.